I think you're over-analysing but a few posters have hit the nail. I was a TRA. Very much "real feminists are trans inclusive!!" "Your feminism is bs if it doesn't include..."
It's as simple as it seeming like the natural progressive step, if you support women's & lgb rights. Naturally you also want to believe that why SHOULDN'T someone have the freedom to be whoever they want to be. And you don't consider shades of grey, not thinking this is akin to being a religious conservative nut that thinks men must remain real men cos god sez so.
Additionally you'll probably in a sjw environment which IS cultish. It's why every person who comes on here is so fucking dramatic and emotional. Do you ever see someone come on here, offer a reasoned calm view without hyperbole?
It's because seeing views like this is terrifying. You are never exposed to it - the outside world is viewed as those who are indifferent and therefore complicit in harm, right wingers/violent men and terfs who for whatever reason are the worst of the worst, and are responsible for actual violence. I think it's because the rw is irrational and no one in the cult would swing that way. But there is rationality in gender criticism and it's basically the voice yelling "the emperor has no clothes" which they really don't want.
So you are never exposed to this view and when you come across it you think it comes from miseducation, bigotry, all you need to do is just read this one article - not realising all the academic lit we've probably read. When you're then confronted with "why did we need to invent words for terms we already have?" "What is a woman?" You ever seen an answer that was unique? Man it's terrifying the answers are always "a woman is anyone who identifies as one" and with any further pressing, the whole house of cards collapses. So they'll either start really questioning and running away in fear to go back to the safety of the cult and the ideas that don't scare them. Because being a TERF is a terrifying thought. No one wants to be responsible for the death of hundreds of trans people, all you want to do is do good.
Only way out is to be exposed to radical ideology and realise you really can't answer those questions. I was always anti porn and very, anti-sex positivity culture. So I read a lot of blogs and lamented how "terfy" they were. I lamented how terfy MN is. But more and more I couldn't answer "whats the difference between transgender and transracial" and the TRA side answers were increasingly flimsy. Now, I am firmly out the woods.
So there you go.