Hmm. Interesting stuff about Piper indeed :)
Though, I saw some Google projects being mirrored to GitHub. Also, fuchsia.googlesource.com/ shows they keep some projects on their own "Google Git" kind of thing.
Maybe that's only true for their open source projects and the internals are kept in Piper as you mentioned
The Chromium and Android people do use Git, but only because their projects are open source and nobody outside Google can use Piper. They do a lot of things with it that are inspired by Piper though, such as copying all their dependencies into the repository in a directory called third_party.
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