Maybe they went to ten linguists, and all they got as an answer is "there is no objective definition of 'fluent'. You are trying to find a single correct result that doesn't exist!"

Then, armed with the naiveté of thinking that if there is something like 'fluency' it must be possible to measure it, they just threw a bit of money at the problem. Note that asking a representative group of people is the closest you can get to exactly what you want to measure (apart from asking everyone). It doesn't matter that there's no agreed-upon method to measure the quality of pizza: if I maximise the subjective impression, I'll get exactly what I wanted.