It's more complicated than that. Initially, there was Perforce. Then, Google grew client-side tooling on top of that ("g4", rather than "p4"). Then, someone grew client-side Git on top of that (for reasons, good or bad). And at some point, the Perforce backend simply stopped scaling, and a new backend was written (named "piper", but not the "piper" most non-Googlers think of). And once that was in place, assorted more "make it scale" tooling could be built on top of that (and, at that point, I stopped using the client-side Git, because CitC was simply That Good).

So, "from scratch" is a bit of a stretch.