> but the co-authored by Claude makes it seem that everything was done by AI

Likely because, in 99% of cases, it was. I doubt anyone would willingly leave the co-author advertisement (because that's what it is, an advertisement) on display in all their commits unless they've gone all-in on the fad and are actively proud of the fact that they're not writing any code themselves.

That said, I don't think this is a bad thing. It helps signal which projects should be avoided if you care about quality at all.