This is absolutely true for real-world coding. However, for the purposes of an exercise, I think the fact that 15 was chosen was not an accident, but is part of the exercise. Does the coder recognize the relationship between the multiples, and recognize the ability to optimize by removing an extraneous comparison?

It's kind of a trivial test for that sort of recognition, but anything as small as FizzBuzz is going to be rather trivial.