And as a bonus they're building all this in Rust, probably the first language to seriously compete with C++ (for the things C++ is good at, like web browsers and game engines, where complexity and speed rule out most other languages).
So creating a new language is not at all that far away from their core competency. They are also working in a space where they have a strong need for a good systems language, so the motivation is there as well.
How about Object Pascal? I think Object Pascal is a better C++ than C++. It's too bad it isn't more widely used. Here are a couple of interesting perspectives from people who have done some recent development with Object Pascal:
1. https://news.ycombinator.com/item?id=11005203 (see also https://github.com/whatwg/wattsi)
2. http://ziotom78.blogspot.com/2015/01/lfi-data-analysis-with-...
I could only find two Rust repositories on their github page, both of which are library bindings forked from other projects, so that seems to indicate some Rust usage. But then again there are more Haskell repositories than Rust repositories: https://github.com/dropbox
Their Rust stuff is closed source, not open source.