Wow, somehow this was the first time I learned that...but then again, Mercurial has never really been on my radar screen as a tool to use. Of course, with your VCS, you get better network effects by picking one that other projects in your language use, not what the VCS is written in. So I guess it's unsurprising.
But now I want to go read through the Mercurial source as I've been curious about VCS internals, but my C-fu is very weak, and attempting to read through the Git source is quite a chore for me.