I'm not an expert, by any means, but I, too, live at high altitude (6500') and I know lots of people that live over 9000'. Lots of indigenous people live in the Andes at 14k and higher. I think the effect they are talking about occurs, to any degree worth worrying about, at much higher altitudes - like 18k+. At least that's my understanding from the little bit of non-off-the--cuff information I have come across.