Perhaps I am missing something but this looks inconsistent to me, i.e. skip_validations should bypass the guardian check either in both cases or in none of them.
In any case, I fixed this because I needed it myself (to have an importer create posts in private categories). Here is what I did:
Je viens de rencontrer ce bug aussi. Très frustrant que vous l’ayez corrigé il y a 6 ans et publié une pull request qui n’a finalement pas été acceptée.