This makes no sense. Sure: you can generate an X.509 certificate that says whatever you want, but the point is that you can validate the signature and see that it's a forgery. In the case of a hash-addressed system like git, the problem is that the signature is over a collision, so it no longer certifies the thing its supposed to certify. Git uses the hash as a shorthand for a revision, including its entire history--so yes, it is using the hash that way.

By that logic, would MD5 be fine? MD4? CRC32?

A digital signature usually signs (~encrypts) a hash of the content. So asking what exactly signing a commit or tag entails is a very valid question. I would expect that signing a tag is only as strong as SHA-1, since a tag is essentially a label for a commit hash. For signing commits I have no clue, but would be quite interested as well.