The claim is there would be some differences, in the short term (dozens of years), and that these would necessarily _become_ huge discrepancies, over the long term (hundreds or thousands of years).  This amplification of difference is inevitable due to multiple nonlinear effects operating in all types of life ecosystems, cultures, etc.  A few thousand years is a trivial timescale when it comes to planetary evolution dynamics and life cycles, which is generally measured in billions of years.

I can see how the “for sure” makes it look overconfident.

Suggest reading the linked-to post. That addresses most of your questions.

As to your idea of having some artificial super-intelligent singleton lead to some kind of alignment between or technological maturity of both planetary cultures, if that’s what you meant, please see here: https://www.lesswrong.com/posts/xp6n2MG5vQkPpFEBH/the-control-problem-unsolved-or-unsolvable

Maybe there is a simpler way to state the idea:.

• 1; would the two (planetary) cultures diverge?  
  • (yes, for a variety of easy reasons).
• 2; would this divergence become more significant over time? 
  • (yes, as at least some of any differences will inherently be amplified by multiple factors on multiple levels of multiple types of process for multiple reasons over multiple hundreds to thousands of years, and that differences in any one planetary cultural/functional aspect tend to create and become entangled with differences in multiple other cultural/functional aspects).
• 3; would the degree of divergence, over time, eventually become significant -- ie, in the sense that it results in some sort of 1st strike game-theory dynamic? 
  • (yes, insofar as cultural development differences cannot not also be fully entangled with technological developmental differences).

So then the question becomes: "is it even possible to maybe somehow constrain any or all of these three process factors to at least the minimum degree necessary so as to adequately prevent that factor, and thus of the overall sequence, from occurring?".  

In regards to this last question, after a lot of varied simplifications, it becomes eventually and finally equivalent to asking: "can any type of inter-planetary linear causative process (which is itself constrained by speed-of-light latency limits) ever fully constrain (to at least the minimum degree necessary) all types of non-linear local (ie; intra-planetary) causative process?".  

And the answer to this last question is simply "no", for basic principled reasons.

1A; re; "on a long enough timeline" and "not a useful claim to make": The timeline indicated in the claim is "up to appx 1000 years".  Insofar as humanity has already been on the earth 100X that, and could presumably be around (assuming the absence of tech) for at least that much longer, the stated claim differential is meaningful.  And then there is life itself, which would presumably last at least another 500 million years, again when assuming the counterfactual of a complete absence of tech.  Overall, this feels like a 'tl;dr', obviating important details.

2A; "Hard to parse" does not directly imply "not epistemically warranted".  To state otherwise would be an equivocation.  To make an assessment of whether 'warranted' or not, would be to have parsed the argument(s), and then evaluated the premises, and correctness/appropriateness of the transforms, etc.  Otherwise, until an actual valid parse has been done, all that can be known to the reader (and observers) is that "claims have been made".

Nor does "hard to parse" directly imply 'not valid' (ie; correctness) and/or 'not sound' (ie; relevance).  Code can be hard to read, and still be correctly executed on a computer to accomplish some practical real world purpose.

Also, 'hard to parse' is a presentation concern, and thus a social issue, rather than a logical issue.  Ie, it is more about rhetoric, rather than about reason, rationality, and truth.  While in social process, both matter, they matter in different and in largely non-overlapping ways.  The concerns stated (and the associated claims) need to be kept separate.  That is merely proper discipline, and anything otherwise is probably just politics (ie, is not really about cooperation in the interest of identifying important relevant truths).  Where possible, certainty is always of interest, for its own sake.  And it cannot be obtained by any form of rhetoric.

1B; In regards to 'strong claims about X' and 'thousands of years into the future', there is nothing inherently impossible, reasonable, or impractical about any such conjunction.  The heuristic of being a-priori skeptical about any claim in that form is often warranted, but not universally.  The interesting cases are the exceptions.  To assume, without additional examination (parse), which such claims are to be rejected without further review is simply to say "I did not read and evaluate the argument" (and therefore do not know for sure), and not that "the argument is for sure incorrect" (based on what, exactly?).

2B; It is in this sense that Remmelt has shown up differently than most: he spent more than 6 months carefully going over (actually parsing) and challenging -- and attempting to reject -- every single aspect of the arguments I presented.  This happened until he had convinced himself as to their merit -- and not because I convinced him.  Like nearly everyone else, he really really did not want to be convinced.  He was as skeptical as anyone.  We both had to be patient with one another -- me to write responses -- and him to actually read (parse) and think up new (relevant) questions to ask.  This history is largely the reason, that he recommends that people read,. and do more of their own work -- it is the adult standard he held for himself, and he naturally has that expectation as a bias regarding others.  At this point, even though no one likes the conclusion, we both feel that it is overall better to know an uncomfortable truth than to remain blind.

3A; You ask Remmelt to "reconsider the infallibility of the person" rather than to 'maybe reassess the correctness of the argument'.  The infallibility request seems to hide another subtle equivocation and an ad-hominem.  We both know that I am human.  What matters is if the argument is actually correct/relevant.  We all know has very little to do with myself as a person.  To implicitly suggest that he is 'deluded' because you are skeptical (in the absence of actual neutrally interested evaluation) is not really an especially 'truth seeking' action.

3B; We are both aware that my style of writing, argumentation, and conversing is not easy to read (or parse).  It is agreed that this is unfortunate, for everyone, including us.  My (necessarily temporary) offer to write at all (and to give of my time to do so -- to assist with others understanding or misunderstanding, and/or to add clarity, where possible, etc) is not infinite, indefinite, or without actual cost, effort, and loss.  So I tend to be a bit sparing with where and when, and with who, I will give such time and attention, and concentrate my "argumentation efforts", in fewer places, and with fewer people. Usually those who have done a lot of their own work at their own initiative, with patience, clarity, discipline, etc, and to not be showing up with various warning flags of motivated reasoning, actions of rhetoric, known logical falsities presented as truths, etc.

My choices with respect to maybe providing rebuttals to any adverse commentary on a public forum, remain my own, on a volunteer basis.  If it happens that I do not choose to respond (or cannot, due to disability), that does not in itself "make" the underlying argument any more or less valid, or relevant, it simply affects whether or not you (the reader, the observer) happen to understand it (as per your own choices regarding your investment of time, etc).  That means, mostly, that if you want clarity, you will probably have to seek it yourself, at your own effort, as Remmelt has done. To at least some extent your skepticism, and how you handle it, is your choice.  We all know that it is all unfortunate, unpleasant -- the whole package of claims -- but that just is the way it is, at this moment, at least for now.  

Adults will do what is necessary, even if it is hard; even if all of the children around them will ever continue to want live in some sort of easy fantasy.