Unfortunately, yes, just about everything beyond the basics in longevity stuff has that as a possible confounder. Sauna is a fairly passive thing, though, I would expect less of an effect.
It’s not only that confounder but also the fact that the studies show massive risk reductions that are really surprising. Considering how much finns apparently go to the sauna compared to similar countries and how good the sauna appears to be based on the studies, it’s weird that they have similar health stats at a country level.
