Interesting perspective. That sounds also true to me.
The idea I was going for—though I may not have expressed it well—is that internet capitalism wants everyone's behavior to be predictable and moldable, and apart from what it can capitalize on, it doesn't care if we have a secret interior sense of self. "GC" transphobes' views seem to align with that framework of human nature: they want to describe things in terms of "male," "female," "pants," "dresses," but apart from that, they'll say "there is no such thing as trans, it's invisible, it's imaginary, you can't point to it, you can't prove it." They're appealing to a behavioristic framework that demands and privileges data. Knowledge of what other people "actually are" (external, shared, quantifiable) gives power to internet capitalists. It seems to me that those who call themselves "GC" are imitating that capitalist demand for knowledge, while rejecting assertions of selfhood that are poetic and vague and messy and essentially private and unprovable. Maybe they speak this way because something about the Google/Facebook era encourages people to think that way, even when a person doesn't personally stand to profit from taking this attitude or worldview.
To return to your point, yes, I agree that a data company may not care if the people in its data sets are cis or trans as long as the company is making money off them. But individual transphobes (those who have nothing to do with those companies) are not making money, and they lack the extensive data sets that Google/FB/etc. have. Yet in some sense their speech resembles how a data company would speak if it were not very good at facial recognition and if it were mad at trans people for resisting their facial recognition attempts. When they say "we can always tell!," they're fantasizing that they're a data company, and they're speaking in this imagined data-company voice for their own transphobic purposes. Like when kids playact at being superheroes and announce that they have superpowers they don't really have. (Actual data companies probably can frequently tell we're trans, which is scary, but also another topic.)
That is my idea I'm floating here. I might be wrong though.
