I think that used to be true, but influencers and such I believe would value some of the freedom of moving to other platforms and keep both their content and follower.

Also, I think many users would now appreciate more control over the moderation policies they want applied, and also be able to choose between different feed algorithms to find one that promotes things that they prefer.

Would most people still probably use the one big "instance"? For sure, but I think you'd still have a good 20-30% that would use alternatives.

Assuming it all just-worked. Which I think is what this article is trying to say, the AT protocol can provide these features and ease of use. I don't know if that's true, but it seems to be the claim.