Well, first of all, Dahlen was countering not the claim "autistic people are not less rational than others" but the claim "autistic people are not extra-rational", which I never said and never meant.
Secondly, while indeed "rationality" is sometimes defined that way, if you take that definition at face value then you conclude that (e.g.) blind people, poor people, short people, and ugly people are ipso facto "less rational" than others. Maybe we need a word to denote "tendency to win" that covers all those things, but I think using "rationality" so broadly would cause too much confusion.
There's more to be said for an intermediate position that takes "rationality" to cover all cognitive skills that tend to promote winning. But it seems like this would (e.g.) lead to the conclusion that if you have two otherwise identical people, one of whom has a slightly better ear for musical harmony, then the latter is more rational. Or perhaps that whether s/he is "more rational" depends on our hypothetical people's social context in really complicated ways (to take one complicated-ish example: if this person is just about good enough musically to be a professional musician but would actually be happier and more productive as an actuary, being one notch better musically might substantially harm their propensity to win overall by making them more likely to choose music as a career). This, again, seems like an over-broad use of "rationality".
Well, this discussion would have gone much better if you'd focused on the different definitions of "rational" (and then maybe discuss which is relevant for purposes of evaluating FrameBenignly's suggestions) rather than calling Dahlen "reprehensible" for even bringing the topic of autism up.
For example, what would be inappropriately off topic to post to LessWrong discussion about?
I couldn't find an answer in the FAQ. (Perhaps it'd be worth adding one.) The closest I could find was this:
However "rationality" can be interpreted broadly enough that rational discussion of anything would count, and my experience reading LW is compatible with this interpretation being applied by posters. Indeed my experience seems to suggest that practically everything is on topic; political discussion of certain sorts is frowned upon, but not due to being off topic. People often post about things far removed from the topics of interest. And some of these topics are very broad: it seems that a lot of material about self-improvement is acceptable, for instance.