Mercurial (with lots of extensions) sits on top of Piper at Google. It doesn't replace it.

I thought it was Facebook that did the mercurial thing: https://code.facebook.com/posts/218678814984400/scaling-merc...

Actually that says they are working on improving Mercurial to the point where they can use it.

That article doesn't claim that. It only claims that mercurial is used within Google.