Maybe, but if so, I bet it's negligible. When it comes to discovery, there's so many places I'd look for FOSS projects before going GH. Except maybe to check awesome-lists, but you don't have to be on GH to be linked on one (and I've seen them popping up on Codeberg). GH's design in general doesn't seem to promote stumbling across new projects. Even if I'm wrong, one could always mirror on GH.

As for contributing, if someone is willing to go though the trouble to contribute, I'd hope they'd go through the trouble of signing up on a new platform. Maybe there's a non-zero number of contributors who would not, and that's an unacceptable for some projects. There's also potential for more contributors if they trust a project is living FOSS principles and less at-risk of vender lock-in. The fosstodon thread shows people care about where a project lives. The arguments in favor of staying on GH seemed mostly inertia-based.