That's how he likes it, to be honest. He doesn't want it to be some statement about Rust or Swift; he left Rust long ago, did some other stuff for a while, and there's only so many "work on a programming language" jobs out there.
In general the Rust and Swift teams are very friendly with one another, and have shared several devs.
Wasn't he the original designer of Rust and employed at Mozilla?
Surprised that this move completely went under my radar