Vibecoders like to make all kinds of things that already exist, often don't care (or notice) if the AI rewrites everything instead of using what exists, and LLMs love to do that.
And nothing actually prevents a vibecoder to clone a repository on SourceHut, they just would be less able to add a ton of crap to it and spit it back on SourceHut (they'll just spit it back on GitHub).
I guess that's also a plausible possibility. Honestly I mostly don't think sr.ht has enough market share to matter either way.