As the "rationale" document points out, implementations are free to do something well-defined in the cases that the standard considers UB. For example, an implementation may document that it detects out-of-bounds array reads and these always return the value "0", and a hypothetical "C" program could rely on that. But implementations explicitly aren't required to do that, hence code that relies on a particular interpretation of UB in a particular implementation is nonportable, since it is a program written in an extended dialect of C, not ISO standard C.

Options like GCC's -fwrapv/-ftrapv and -fno-strict-aliasing are examples of language extensions that are essentially implementation defined UB.

Edit: Of course you could argue that things where hardware difference are a likely motivation such as signed integer overflow ought not to be UB in the first place, but instead left as implementation defined in the standard, but in that case your issue is with the C standard committee, not with implementers.