As for the second quote, the surrounding context explains that the validator will by default return an error if you point to a single object using two different pointers with different opinions on what the pointee's type is. This doesn't sound like a safety issue, since the validator is being too conservative rather than not conservative enough.
The first quote is probably in part referring to the second quote. If that is all it is referring to, than there is no safety issue. If there are other similar issues but rkyv chooses to reject valid archives rather than accept invalid ones, then there also is no safety issue. However, that isn't unambiguous, so I can't say for certain that it isn't possible to misuse the library from safe rust.