I don’t think most real women would bring up the fact that they're a woman - especially on a car forum. Not that they’d hide it, but just wouldn’t bring it up, the same way most men don’t bring up that they’re men. When someone makes a point of mentioning it, it immediately raises suspicion, and if they get defensive about it, that’s another red flag. Throw in a gendered username or an avatar on top of that and you’ve got all the hallmarks of a guy pretending to be a woman.
I know some women are genuinely into cars - I even know one personally - but that doesn’t mean they can’t act like men. And most men have been fooled enough times online that we’ve just gotten pretty cynical about it.