> That might well change, but it's what their docs currently say.

It's not, actually: https://source.chromium.org/chromium/chromium/src/+/main:doc...

> Rust can be used anywhere in the Chromium repository (not just //third_party) subject to current interop capabilities, however it is currently subject to a internal approval and FYI process. Googlers can view go/chrome-rust for details. New usages of Rust are documented at rust-fyi@chromium.org.

It is true that two years ago it was only third party, but it's been growing ever since.