Kind of anecdata here - I live at 1300m all year round, and get a reasonable amount of exercise throughout the year (20min walk every day, cycling in the summer, snowboarding in the winter most weekends). So I guess I fit somewhere between the "Altitude plus prolonged exercise" and "No altitude plus no exercise" group ("Some altitude plus some exercise"?)

Anyway, at the start of the ski season every year I go to the glacier and snowboard there, as they always open a couple of months before my home resort. The glacier is around 3000m and requires a reasonable amount of effort to get around. Usually the first couple of trips there result in me not feeling that well, occasionally I might really feel bad enough to call it a day. I definitely can feel the affect on my body/mind.

In a way I guess that agrees with the last paragraph of the article.