Yes and no. It's true that there's some portion of the kernel space code that can't be written provably safely. But there's good reason to believe that a lot of it can and that belief is what's driving this integration.

Whether Rust in the kernel succeeds or not will likely be determined by whether or not a sufficiently clear boundary can be drawn between the bit that must be unsafe (in the Rust sense) and the rest. And how much code is in the latter. I don't think we know the answer yet but some knowledgable people are willing to run the experiment on the basis that the probability seems quite high that a safe subset can be determined.