As Aral said, it probably has a bit a true and falsness in it. Higher altitude does mean thinner air, but also less O2 and more pressure on the lungs. Now that ive simply restated aral let me expands on it :/
It would be the ratio of the thin air: O2
ressure. i would imagine that at a higher altitude your smoke is forced to be more concentraded as it travels through your lungs, resulting in thicker clouds, espescially in the thinner air. so it probably is true that the rings are better. However at higher alitude i'd bet he cant pull nearly as much volume of smoke he can at sea level. ive actually kinda done this before now that i think of it. I live at sea level, but i go to a friends cottage up in the mountains.