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.
As with most of these little tests, the actual question is, "Can you explain your decision, and does your explanation make sense?"
Really, the FizzBuzz test is a check to see if someone is bullshitting when they say they can code. Testing deeper than that is expecting too much of it.
It's kind of a trivial test for that sort of recognition, but anything as small as FizzBuzz is going to be rather trivial.