Its both. At sea level, that engine could produce insane power, but up there its just enough. The tough part is actively tuning it for climbing into the upper atmosphere. The output/metering at base (16,500ft) and at 29,000 ft are very different. If they tried to run it at 100% at sea level, or even at 10,000 ft, it would snap the blades off.
They've been testing this thing for quite a while out there, and fianly found a day calm enough that they could summit. Some of the guys on Summitpost have written about seeing it during testing, refering to it like a UFO in Nevada, but with more suprise. "WTF is that sound?....WTF is that doing up here!?"