What if a future race comes along in a million years and finds the remains of Yao Ming?
The odds that they will do so, though, is astronomical. It's a virtual certainty that the remains they find will be from humans of rather average size, so basing a tentative conclusion about the height of a population from one individual isn't as outrageous as you make it seem.
Nobody who does that pretends they have anything definitive, but establishing a tentative range from one individual isn't as invalid as you might think.