They stole my idea
Okay, maybe not, but I swear I had that hg absorb idea a couple of weeks ago. I use Git almost exclusively though. I've used Mercurial on the odd rare occasion for free software projects I was contributing to but I never quite got it. I think a lot of it simply boils down to a difference in terminology. A little competition between the two is healthy.
As for Rust, I haven't taken the time to learn it yet but I have had a gut feeling that it could go on to do great things. This work certainly sounds like it's being put to good use.