I believe the claim is that there's nothing in the C standard that requires implementations to be unsafe. If they wanted to, they could bounds check pointers, check allocations are still alive when pointers are dereferenced, etc. and still be conformant to the standard.
Nothing in the C standard requires bytes to have 8 bits either.
There's a massive gap between what C allows, and what real C codebases can tolerate.
In practice, you don't have room to store lengths along pointers without disturbing sizeof and pointer<>integer casts. Fil-C and ASAN need to smuggle that information out of band.
