The linked reference page does not say that implementation-defined behaviour must be sensible, only that it must be defined. Contrast with unspecified behaviour where "Each unspecified behavior results in one of a set of valid results."
I expect that most instances of implementation-defined behaviour come with additional rules which state that the implementation has to define something sensible.
I expect that most instances of implementation-defined behaviour come with additional rules which state that the implementation has to define something sensible.