"Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message)."
Ignoring the situation completely means exactly that: completely. The situation is not being ignored completely if the compilation of something which follows is predicated upon the earlier situation being free of undefined behavior.
OK, so since the situation is not being ignored completely, and translation or execution is not terminated with a diagnostic message, it must be that this is an example of "behaving in a documented manner characteristic of the implementation". Well, what is the characteristic; where is it documented? That part of the UB definition refers to documented extensions; this doesn't look like one.
What is "characteristic of the implementation" is in fact that when you multiply two signed integers together with overflow, that you get a particular result. A predictable result characteristic of how that machine performs the multiplication. If the intent is to provide a documented, characteristics behavior, that would be the thing to document: you get the machine multiplication, like in assembly language.