I thought before clicking they were going to announce that the X Window replacement was being rewritten in Rust, lol. They should change the name.
This isn't really a user-facing feature, so name clashes aren't as important. Context should be more than enough here.
I don't think they're concerned overly much with the marketing presence of a compiler implementation detail. :P
One name is in all caps, the other is not, and it's easy to add another word to your website searches to get the one you want (e.g. MIR Rust, Mir Ubuntu).
Yeah, there seem to be a few names that are overused for completely different projects. Mir, Dash, Unity