I don't doubt what you're saying is true, I have heard similar things many many times over the years. The problem is that it's always stated somewhat vaguely, never with concrete examples, and it doesn't match my (perhaps naive) reading of any of the standards.
For example, I just checked C99[1]: it says in many places "If <X>, the behavior is undefined". It also says in even more places "<X> is implementation-defined" (although from my cursory inspection, most -- but not all -- of these seem to be about the behavior of library functions, not the compiler per se).
So it seems to me that the standards writers were actually very particular about the difference between implementation-defined behavior and undefined behavior.