Regular Rust assumes that you use unsafe in a proper manner, but still asks you to mark functions that can cause memory unsafety with the wrong inputs as unsafe. This is so people know what code to audit. If you don't do this (or at least, mark something in the module as unsafe), it's considered a bug in your library. Among other things, this kind of clear isolation boundary makes it obvious where your proof obligations are and consequently which code you can ignore when debugging undefined behavior (code outside the unsafe module is not responsible for the unsafety no matter what weird input it provides).

This build tool, as first proposed, knowingly exposed macros that can cause memory unsafety with the wrong inputs, without making you write unsafe. If a build tool makes assumptions like "you probably know what you're doing with that C++ code" without asking you to write unsafe, it is just as wrong as an implementation of unchecked indexing that wasn't marked unsafe (because "you'll probably know not to use this unless you're really in bounds."). It is clear that you consider this a trivial matter, but many people very heavily involved in the Rust community (including the people working on its soundness proofs) do not.