> Combining this with the fact that Everest climbers are apparently eating 10K+ calories a day and then up to around 20K on summit day, I have to wonder how these facts jive...

The estimate is probably off for extreme circumstances and those people probably aren’t fully digesting and using all of those calories. The accurate number would be interesting.

I too have found that cardio isn’t very good for dropping weight, but packing on some muscle does a lot to shift body composition. Maintaining more lean mass simply requires more calories.