Google replaced perforce with Piper, a perforce compatible system written in-house. This system was not based on git.

There were systems that allowed you to mirror portions of piper into local git repo, giving you a git-like experience on top of piper, but the underlaying system was piper and not git. (At this point my knowledge is 4 years out of date.)

Git? I thought Google uses a custom system?

We do. It's called Piper: https://cacm.acm.org/magazines/2016/7/204032-why-google-stor...