It's probably because there ain't much else to do up there than just train a lot. They did have some people along too, there was a guy with Mas Oyama for several months the first time he went into the mountains as I recall.
Here's an aside of mountain life I read in a history of the Gurka's book. They spend their life gaining superior fitness walking up the slopes and steps, but cannot run in a straight line on flat ground when taken to England!
I can vouch for Gurkha and Sherpa fitness, our Sherpas in Nepal were awesomely fit, carrying about 70kilos (150 pounds) from one strap on their heads, at 4000/5000m in flipflops.
how is that not impressive?
I didn't get much time to train, as I was walking all day most days. but despite taking several months off training, I came back much better, simply due to being acclimated to high altitudes.