The risk of using mirroring rather than versioned backups is that you lose all the data when a deletion is mirrored.
Redundant storage is not backup.
And even that is because of a deliberate decision on the sysadmins' parts based on a misunderstanding of how git clone --mirror responds to a corrupt repo, not some simple oversight. Which is to say, countermeasures will be put in place for that as well.
I do wish people understood why having relying only on even 2-week-old backups is unacceptable in the context of a large active FOSS project's source code repository, it's not like it's OK to simply start over again from KDE 4.9.4.
Yes, but i'm not convinced you see that this is EXACTLY what you are exposing yourself to.
What if next time it's a (nasty) bug in git? A push causes corruption perhaps?
Drop the idea of using git itself to host the backup strategy. Switch to plain old backups, if space (or performance - i'd wager the kde git repos must total a good few hundred GB, if not more, if there's artwork or other binaries in there too) is an issue there would be nothing wrong with incrementals for the */30 min backups.