I don't know what the global statistics would be, but going up in altitude and exercising is not an unusual event. As I mentioned earlier, this type of altitude and climate change happens every weekend in the Phoenix valley with people heading into the higher lands for recreation. And it's not just skiing, but hiking, trail running, road bikes, mountain bikes, kayaks, horse riding and rock climbing. Some are more cardio intensive than others, but are similar to the circumstances of the OP. Humphreys Peak is at 12,500, and folks from the valley hike and run it every weekend. I'm sure there are many other similar circumstances around the country.
In the case of the OP, I don't know how he died, but presume it wasn't from pulmonary or cerebral hemorrhage, which are the classic signs of altitude. So it seems most likely that his activities that day put enough of a strain on his body to expose an underlying weakness or condition. Not sure why this theory is causing such resistance, it is a medical scenario encountered daily. Perhaps the most well known being the "snow code", when people, apparently in good health have sudden cardiac death shoveling their driveways.
When you have an uncommon outcome from a common experience, you have to consider secondary causes. That's all I said and I'm not sure when such a simple statement would meet with such resistance. Perhaps I'm too new to the group and haven't earned my right to have an opinion?