I believe the article over-sells Fuchsia's use of Rust. Raph Levien wrote some bindings to the OS runtime, and he does work at Google, but his Rust work is not-official.
Which is a shame, they are missing an opportunity to ditch C/C++ in favor of a safer language and set a precedent in OS history.
Imagine how easier contribution would be if you could write the OS parts with less lines, guarantied to no introduce most security and concurrency bugs we know about.
(or that's the story as I remember it)