> cryptographically unique

Git uses SHA1 which is pretty far from cryptographically secure. It’s unlikely that’s the sole reason for having a separate checksum database though.

Doing some searching around online, it seems like git introduced sha256 support several years ago.

Only for some local operations- git protocol still uses SHA-1 for compatibility with old clients, and existing SHA-1 objects in old repositories aren't converted.

Git history can be rewritten or deleted e.g. by the git rebase command. The mirror's checksum DB is independent and prevents your build from breaking or being altered if a dependency is manipulated or deleted.

As sibling comment notes, Git's hashes are not secure: https://shattered.it