Yeah, that's a sticky one as people can do the exact same thing naturally. Around here it's quite east to live in a town like Gold Hill or Ward (~10k feet) then ride down the canyon in the morning, do flat/hilly training all day at low altitude (~5280 ft) then do a bit of climbing back home and sleep high......

Then there's Mauricio Soler who's been training at 14k ft back home and just laughs at the "high altitude" of the Alps.