I don't agree. I think a module operator is common enough that it should be known, and even if they don't know it, they should be able to quickly make a module function. (Something like a while loop that decreases value a with value b while a>b and returns a)

You say you don't agree, but you don't disagree either. Perhaps you should know a modulo operator but that's still not what FizzBuzz is for. It's to see if the candidate has the most basic concepts of loops and tests.

Adding knowledge of a relatively obscure operator (can't remember the last time I needed it in real life) just makes it a trivia test.

Asking a candidate to describe 2 different ways to do modulo is an excellent interview question, imo.