I think GP is using 'semantics' to refer to the emitted code, which is "simple" in the sense that it e.g. doesn't have a runtime or a garbage collector. What isn't simple is all the features of rust that are unheard of in other languages, like the borrow checker, but that is a feature of Rust's syntax, and (I am not a Rust expert, but my understanding is that) many of them "vanish" when the Rust code is compiled.