I'm not making that assumption, I'm asking what factors they included. Lots of studies are poorly run and fail to account for very simple confounding factors.
The big study was a 20+ year observational study in Finland [1] where Sauna's are supposed to be quite accessible, but the world is complicated and there are probably many confounding factors.
The mechanism of action seems very plausible. Sauna's are moderate stress which spikes your heart rate temporarily (they say 100-150bpm in the study). So it's not surprising it could be protective for cardiovascular disease just like many other forms of moderate stress/exercise.
