Except lemmy specifically is AGPL and it’s basically impossible to monetise as a startup because they can’t close the source code.
https://github.com/LemmyNet/lemmy/blob/main/LICENSE
Kbin too:
https://github.com/ernestwisniewski/kbin
They’d have to create their own from scratch.
That’s not the end of the world, though it does mean that a competitor could always start using it. I was going to say just use Lemmy but a major site would probably want to have their own fork for stability so they’re not at the whims of someone else.
They could probably use an open fork for a while while also developing their own software that would be compatible and then seamlessly switched out.
Which is fine, ActivityPub is an open protcol. You don’t even need to cleanroom, its all there. The AGPL is a shield against corporate interests, given how things are going that’s a much needed feature.
There’s also strong opinions if open source instances should federate with closed source instances, for reasons of https://en.m.wikipedia.org/wiki/Embrace,_extend,_and_extinguish
