You have two points confused -
> To you, it's obvious that free speech does not lead to truth-seeking.
> free speech is necessary for truth-seeking
These are not in conflict.
It is necessary that football-kicking be legal for football (soccer) to take place. However, football-kicking being legal does not necessarily lead to soccer, and may in fact lead to all sorts of other ball-kicking related activities.
Yes, freedom of speech is indeed necessary for the free exchange of ideas, and for the concept of a marketplace of ideas as envisaged by such philosophers to exist.
That doesn't mean that the marketplace of ideas that they envisage actually exists or functions as they would imagine. Specifically we see the public commons becomes dominated by commercial and politically driven messaging, often both made and propagated in bad faith. The idea of a functional marketplace of ideas, of "truth-seeking" as you put it, requires honest actors for whom this truth-seeking is a goal. That turns out to be fine in the context of academia, but wider public discourse doesn't operate that way.
> What concerns me is not social media moderation (though I'm generally against it) but that people like you, who have rejected the standard arguments for free speech, actually have no rationale for supporting free speech at all.
Of course it concerns you, because for you free speech is a point of religious faith. You've already said you'd rather see society fail than compromise on it. It looks like you'd rather have that than even examine what it is you mean by free speech and why that's worth protecting.
Because I absolutely believe the principles of freedom of expression are worth protecting. I just disagree that trolls and shitposters getting banned from twitter is the slightest bit relevant, or that all we can do in the face of campaigns of lies and disinformation is to throw our hands up and say "Oh well, free speech", while attempting to argue with people who are not the least bit interested in truth-seeking or arguing in good faith.
The irony of course, is we're having this discussion on a heavily moderated social media site...