No. People who have contributed a lot can get nominated and if no one raises objections, they typically get the commit bit.

It is less than ideal. On the other hand, self-reviews/merges are frowned upon, so there is normally an extra set of eyeballs that checks changes. Besides that, nixpkgs has a really large number of non-committer contributors. Packages often get touched by multiple if not tens of people. So, even though Linus' law typically doesn't hold up, I do think it holds up pretty well for nixpkgs.