Mercurial was pushed internally as being the "better" (for some dimension of better) between it and Git back in 2010, but I think even the most hardline Mercurial fans have realized that in order to meet developers in the middle in 2015, we need to use Git for our open-source releases. We have a large investment in Gerrit [1] and Github [2] now.
So the Mercurial comment is probably entirely based on scaling and replacement for the Piper Perforce API, rather than anything externally facing.
[1] https://www.gerritcodereview.com/ [2] https://github.com/google