Being out online isn't the same as being out IRL isn't the same as transitioning. More specifically, each of those puts the person in more danger of violent transphobia. Tumblr fandom is a very low risk place to test out their gender identities. Some may realize that this is what's right for them, and slowly work that into their actual life, or realize their identity as a woman has more wiggle room than gender essentialism originally thought, as was the case with me. Yeah, there are probably a few sociopaths who do it for points, because the internet is very large and has next to no rules, but that's far from the default, IMO.

It would benefit you to read up a little on nonbinary gender and gender presentation- off of tumblr, because they can get screamy and radical to the point of absurdy, which can undermine a useful societal concept. It's mainstream enough nowadays there should be some decent sources easily found on Google.