I am quite puzzled by the first paragraph of this post, because even after reading the rest of it, I still find them perfectly meaningful. It appears to me that the post not only fails to defend the idea that the three questions don't make sense, but it actually answers them, just as follows:
"What is the definition of Effective Altruism?": The action or habit of asking oneself the question of how to do the most good with the available resources, using a particular methodology (as per the author's comment on Robert_Wilbin's reply below). "What claims does it make?" : None, as per the definition above. "What do you have to believe or do, to be an Effective Altruist?": Asking yourself the question above and trying to answer it using the given methodology. I have my doubts on the answer to the first question. The term EA often seems to be denoting a social movement or a philosophy, rather than a habit.
But more importantly, when the author and some other people in the comments talk about the "definition" of EA, they appear to refer to something akin to a definition in a mathematical theory: a succint but correct and complete statement describing EA that can later be used to draw conclusions in arguments about EA. However (if you have read mathematics textbooks, you may be familiar with this) such definitions are often insufficient in order to get a clear and complete idea of what something is.
In other words, if somebody asks in good faith what EA is, a much more illuminating reply would not only frame EA as a social movement, rather than a personal habit, but it would note that the general tendency in this movement is to understand the idea of 'a better world' in line with some brand of consequentialism, and it would also describe the kind of "actions" that are typically considered by EAs, such as donating to charities, choosing a career, etc. This can still be done succintly, and yet a much more accurate picture of EA is transmitted.
My point is: it can be misleading to think of EA as being asking a question; instead, one has a more accurate picture if they think of it as a movement, based on a key question and methodology, but also with its correspoding history and particularities. The post, however, does a good job in reminding any aspiring effective altruist that there is a possibility that particular conclusions about how to do the most good which have been determined by the EA community could, in principle, be wrong, and that the only certain invariant of EA is the question (and methodology) of how to do the most good with the available resources.
Just a small note: the book you refer to, "A Plague of Locusts", is actually called "The Locust Effect" (Haugen and Boutros) - took me a while to find it, so figured that others might appreciate the correction :) Thanks for the recommendation.