Jim, I think there are two issues being confused here: acclimatization to altitude and resistance to high altitude syndrome (altitude sickness). I agree with you that there is no documented effect of fitness on a persons susceptibility to "altitude sickness". But, the OP seemed to be talking about acclimatization, not altitude sickness. And, from what I have read, and from what I have experienced, there is a definite link between a persons aerobic fitness and their ability to perform physical activity at altitude.

Acclimatization is the body's response to lower oxygen pressure in the atmosphere. The blood of a person adapted to high altitude has a higher red blood cell count and other differences that show up in lab tests. I have no idea what causes altitude sickness although I have read that there are similarities between it and migraine.

I had an interesting experience when I lived in Flagstaff. I had traveled to Washington DC and became ill there. I visited a physician who took a blood sample. When the results came back she was quite alarmed at some of the reported results, especially the RBC. When I informed her that my home was a bit over 7000' altitude she relaxed although she also said she had never encountered an altitude-adapted individual before.

I, too, have seen exceptionally fit individuals devastated by altitude sickness while far less fit individuals are just fine. But, altitude sickness aside, the fitter one is, the better they will perform at altitude.