Trigger warning: racism.
I personally found this letter incredibly difficult to read. Beyond the content of the email, the apology is also terribly written, and reads like Nick, an intellectual leader in EA and longtermism, might still hold these views today to some degree. It also reads like Nick is primarily just trying to do damage control for using a racial slur, or preemptive PR work for some other reason, as opposed to focusing on the harms he may be contributing to, and the folks he is apologizing to. In this context, this also sounds like a dog-whistle:
Are there any genetic contributors to differences between groups in cognitive abilities? It is not my area of expertise, and I don’t have any particular interest in the question. I would leave to others, who have more relevant knowledge, to debate whether or not in addition to environmental factors, epigenetic or genetic factors play any role.
Myself and other EAs I know are worried about professional reputational risks of continued association with the EA movement or longtermism. This is not just a PR risk, and despite my view that this reflects terribly on Nick and any comms experts who may have been involved in this, I don't want this to imply that the PR angle is what we should be primarily concerned about here-it isn't! But whether or not one of the leaders of EA has held racist views for decades, and whether he still basically holds them today is important.
It has real implications for the movement's future, including selection effects on people who may become more uncertain about the views that intellectual leaders of the EA/longtermism movement hold (and by extension, its intellectual foundations), whether EA is a community for "people like them", and whether EA is a movement that is well-equipped to preserve a future for all of humanity. Even if they aren't uncertain, they may be more reluctant to take risks to continue or become more outwardly involved in an increasingly controversial social movement. This may also affect the view of current and prospective donors to EA causes.
These are not concerns held solely by "EA outsiders" or those who are already unsympathetic to EA.
Reactions on Twitter-read on at your own peril!
(The EA forum seems to default to strong-upvotes on your own posts. I don't know why this is, but I'll probably change mine to a normal upvote if this post gets some engagement.)
Doesn't the first sentence of his old email make this part fairly clear? It sounds like he's talking about the classic edgelord thing of enjoying the tension between intuitive repugnance and (what he took to be) logical truth on a strictly literal reading, when divorced of all subtext (which is presumably not what any reasonable person would ordinarily take the claims in question to communicate). Perhaps similar to how many philosophers find logical paradoxes invigorating. (Cf. Scott Alexander's classic post on related issues.)
That's not to defend it, and I agree his apology isn't sufficiently clear about why his particular example was so egregiously poorly-chosen. But it does strike me as most likely stemming from neuro-atypicality rather than racist intent, for whatever that's worth. (Many understandably care more about racist effects than racist intent, but I mention the latter here since you seem be to be asking about Bostrom's motivations, and that does seem relevant to assessments of blameworthiness.)