> where's the idea coming from that that consensus must be adopted by everyone else as well?
Well, for two reasons.
A) the marketplace of ideas is explicitly designed around the concept that some ideas win, at least at a broad cultural level. Free speech advocates aren't trying to build a world where every idea is treated with equal respect everywhere, they're trying to build a process by which, in general, we move closer to truth[0]. That doesn't mean that the ideas can't resurface in the regular marketplace, but it does mean that for ideas to be taken seriously by the broad, regular population, they have to actually convince people.
The anti-trans arguments I see online are mostly just really unconvincing to anyone who's done marginal research on trans communities. I'm not an expert on trans communities, I don't have a lot of diverse friends -- I shouldn't know more about trans arguments than the average TERF does; and yet I regularly find that I do.
To look at it from a broader market perspective, is it a failure of the marketplace that none of the restaurants on my street sell ice cream with ketchup and green beans? Or is it a success of the marketplace? The marketplace demands that you be able to actually sell what you have, you have no moral claim to force the marketplace to provide you with customers.
More importantly,
B) the consensus doesn't need to be adopted by everyone and hasn't been adopted by everyone, but private communities aren't required to engage with ideas that aren't furthering their own conversations/goals. And for better or worse, because some of those private communities are very attractive to the broader public, those communities exercising their Constitutionally protected Freedom of Association means that there are fewer spaces that contrarians can join.
But again, that's kind of how the system is designed to work. It's contradictory to say that in a free market, people shouldn't be able to choose where they shop. It's contradictory to say that in a free society people shouldn't be able to choose who they associate with.
The irony is that it's the far left candidates like Warren[1] who have had by far the most sensible, market-friendly response to the problem of people being deplatformed. If Facebook/Apple is too big, break it up and enforce antitrust. Don't try to nationalize it, or dictate how people can speak, or get rid of fundamental protections like Section 230. Just get rid of the monopolies and let communities compete for members.
Outside of solutions like that, I am very skeptical of any argument that claims to be in support of free speech that doesn't think protesting, boycotting, and public criticism are also speech. A lot of cancel culture falls into those bounds: I see people who are angry about boycotts, public criticism, markets responding to consumer demand, private communities like Twitter choosing who they host. Most of that stuff is free speech.
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[0]: And there are other goals as well, of course, but that's the main conceit of the "marketplace of ideas" argument.
[1]: Doctorow also has some interesting ideas here about revising the CFAA and breaking down some barriers for building competitors, but that's a separate conversation.