I should clarify my comment by saying monorepo-related Mercurial improvements benefit not just those monorepos backed by Mercurial, but also where Mercurial is "just" a front end to a different system. I mean just think about it, what does a front end in this case really mean? When you run a command like `hg log` how much of the original Mercurial code are you running? Do you design an entirely different system that happens to share the same command-line syntax as the original Mercurial, or do you emulate the .hg folder format and run the original Mercurial code, or somewhere in between? Thinking about this problem would shed more light on why Google's work on Mercurial benefits everyone else with a big monorepo even though Google is "just" using Mercurial as a front end.
And both Facebook and Google have contributed to Mercurial on this front, though admittedly Facebook did more work than Google did. I know the tree manifest feature (https://www.mercurial-scm.org/wiki/TreeManifestPlan) is done by Google and upstreamed, and it benefits every repo with millions of files. (Just clone the hg repository and search for commits with a google.com author email and see what kind of commits they are.)