I think what they're saying is that C compilers don't care about aliasing here because it's not actually bounds checking the pointers, it's just a numeric comparison between two random arguments. It's much easier for an optimizing compiler to eliminate a duplicated boolean check between two numbers this way because passing numbers from one function to the next has no aliasing concerns.

This is a misunderstanding of what's useful about aliasing information. A lot of C code can't be autovectorized because it can't tell that accesses to two arrays don't alias, for example. Similarly it often can't reorder / eliminate redundant updates to arrays because it can't tell they're not aliased. These aren't related to bounds checking specifically but they are things that can improve performance in Rust over C (theoretically, anyway; in practice LLVM is still very much tuned for C so it doesn't take advantage of a lot of this stuff yet, but it likely will in the future).