Lots of people don't know of % works. Just as a lot of C / Java / C# programmers do not know the difference between "if a() & b()" and "if a() && b()"
Funny you say that. And incredibly sad too.
I have participated in interviewing many junior/mid/senior developers and you are correct that they trip up on the % operator. I've also experienced them tripping up on the word "modulo" and instead trying to calculate the remainder of a division operation by hand.
While not trying to be elitist, I'm not sure what it says about our industry when a fundamental concept such as modulus is not understood by programmers.