I'm not sure which shadows which here. It doesn't seem like writing in this style will cause someone to post on LessWrong, while conversely it seems much more likely that someone who has been posting on LessWrong for a while will adopt this writing style. Thus, ISTM that the writing style overshadows posting-on-LessWrong more than the other way around.

Given that I have the text itself, learning that it was posted on LessWrong I wouldn't infer much from it, since regardless of gender they aren't obviously more likely to write this way on LessWrong if they're a regular user, whilst if I only know that someone is posting on LessWrong then learning that they write in this manner also gives me all the other non-LessWrong writer data.

Not sure if I'm really being clear. Basically, in my model the causal chain goes the other way around.