I'm not an expert on the Java specification, but my understanding is that Java as a language is well-defined enough that we can say things like "Java's type system is unsound", independently from any specific implementation of Java.

You can't do that with Rust because "Rust's type system" is literally just the vague concept of "a type system that is safe". The implementation of this ideal in rustc can have flaws, but Rust itself cannot, because Rust itself has no flaws by definition.

but it isn't claiming to be saving you from things it can't prove it can either.