Yes but it's easy to take this too far and conclude that e.g. Javascript is not memory safe because browsers are written in C++ and they have to interface with the kernel which is written in C. At some point you simply need to trust that the current implementation is correct and bug free. This is also a problem with formal verification. What verifies the verification?
Yes, of course you can. But that's not what Animats said. Animats specifically said "unsafe function," which means you need to use the unsafe annotation to call it. See my other reply for more elaboration: https://news.ycombinator.com/item?id=24028359
The point of the GP was that any safe code using this safe API could in fact be memory unsafe is there is a bug in the unsafe implementation.