See the section "Safety measures make memory-unsafe languages slow"

It will most likely be C/C++ -> Rust for performance critical modules.

Much of Android is written in Java/Kotlin. Transitions from the JVM to C/Rust and back are not going to be free due to extra wrappers and the inability to optimize across the boundary. So for some usecases Java will be faster. But anything CPU or memory intensive will probably be cheaper in Rust.

I'm assuming that's why they have Rust in the mix there, for where performance is still critical.