It can be described as a hobby project only in the sense that his employer would probably prefer that he spend all his time working on Firefox.

Tools to do X better are often designed by people who get paid a lot to do X and worry about losing their job if they are not good enough at X.

If he were to tell me that he didn't imagine Rust's helping with browser dev when he designed Rust, then I'd believe him, but the "circumstantial" evidence points strongly in the other direction.