For Fizzbuzz purposes, you only need to know what happens when both numbers involved are nonnegative, which so far as I know is consistent everywhere.
(I deal with the other cases by making sure only to use % or similar on nonnegative numbers. If I ever encounter a case where I need to take remainders and performance is so important that I can't afford to do that, I'll double-check with the language documentation and leave a comment saying what the actual behaviour is. That's never happened yet.)
It's pretty common for people to be very focused on the trivia they know as a litmus test for judging other people, while dismissing trivia they don't know as irrelevant.
I don't have concrete reasons for thinking so, but my general impression is that this attitude is significantly more common in IT than other environments.
(I deal with the other cases by making sure only to use % or similar on nonnegative numbers. If I ever encounter a case where I need to take remainders and performance is so important that I can't afford to do that, I'll double-check with the language documentation and leave a comment saying what the actual behaviour is. That's never happened yet.)