Microsoft PM here. It's important to note that Hg also had similar constraints. So Facebook was in a position of having to fix Git or Hg to suit their needs. It turns out that for a variety of factors, Hg was the better choice for them to hack on.

Microsoft took the same look and went the opposite direction, and decided that Git was easier for us to hack on. This was a no brainer for us. We build multiple tools that host peoples Git repositories (TFS and VSTS) and have Git repositories as deployment mechanisms for Azure. We have several contributors to tools in the Git ecosystem on staff, including people contributing to core git, and the maintainer for Git for Windows, libgit2 and LibGit2Sharp. But we have comparatively little institutional knowledge of Hg.

That post you linked is awesome. Facebook has done a lot of really impressive work scaling Hg and a lot of the lessons learned at Facebook and implementations in Hg are very similar to what we've done with Git.

This video from Durham Goode on Facebook's version control team (from Git Merge, amusingly) is also awesome: https://m.youtube.com/watch?v=gOVD-DrUpwQ