There's some nuance here. Even the most battle-tested system, with distinct slots for executable code and primitive values,
might have a way in which an untrusted primitive input can overrun a buffer, or be split in an unsafe way, and cause unexpected behavior. But there's a vast difference in attack surface between that, and "just give us a string, don't worry we'll sanitize it on our end."
It's all about defense in depth. Even a system that's tested all the way through with Coq or similar is still at the mercy of bugs in the specification or in underlying system libraries. But intentional API design can make it materially less likely that a security issue will arise, and that's worth a heck of a lot.
