Chromium is so incredibly complex, they deal with twenty layers of abstractions and there is concurrency everywhere. Sprinkling a bit of Rust across the code base, and only at the lowest level, will do very little.
(Since their new exploits tend to be in big new APIs that relate to new flags, I think they would be well served by using rust for this splattering of new apis.)
It's exactly the sort of thing this post is talking about. I'm not advocating for a wholesale rewrite, that would be ridiculous - but there's very obviously certain areas of attack surface that are harder to sandbox and are great candidates.
Besides, Firefox is even older than Chrome, and at least as complex, and they began the process of replacing core attack surface. Google could put 10x the devs on such a thing.