You only need one account on any instance and you can rate/comment on any video in the federated network. However there are instances that don't follow each other and for those you would need a separate account.

Surely that's most instances? I don't see why the Blender instance would follow the Extinction Rebellion instance for example?

And doesn't that make it an N^2 problem? Does every instance have to follow every other instance to make this work?

Actually, with v3, you can follow "any" peertube channel or account regardless of whether your instance does the same or not. Earlier this was only possible with a mastodon/pleroma account only. Now your peertube user account in instance "a" can follow any channel anywhere in the fediverse.

This is like you want to follow some channel from tilvids.com but dont want to follow the entire instance.

Right but you can surely follow channels just with RSS? Federation is only required for commenting and voting.

yes. you only need an account in one "instance" and you can comment or aggregate videos from any participating instance and comment on the videos as if you had a local account there. Social function is a big deal of activitypub

What I don't understand is why they don't broaden the scope and work more like IPFS, with a thin shell for displaying video data.

What would IPFS solve that is still a problem today? The only disadvantage of bittorrent is that it doesn't handle mutable content, but 1) videos never change and 2) any mutation of content implies a fixed identity, and that is what Peertube is here for.

The biggest problem to solve is diacoverability, a thin shell is not enough if you want to solve the general problem

Isn't that more or less the idea behind LBRY?

nope lbry uses their own custom torrent-like protocol on top of their own fork of bitcoin with their own weird python lbry daemon which communicates with a javascript frontend which is pretty much completely centralized in the browser version. Even the desktop version relies on centralized features like comments system.

Yeah I think so. Instinctively I do believe that this would be the most natural development. However I thought that federated social networks where kind of cool so maybe this is a better idea.

I do believe I tried to sign up for LBRY but I don't know if it really took up.

Might it be that LBRY simply doesn't scale or that there is some network effect that causes it not to work unless there are some specific conditions.