I can definitely turn my shower up for a hotter body experience than any sauna I've ever been in, and obviously it's nowhere near 150-195 which would be lethal.

Because the water is making direct contact with my body for much more heat transfer. The dry air of a sauna is vastly slower, plus you're sweating which provides evaporative cooling which isn't happening in the shower.

So I too really don't see how saunas are appreciably different from hot showers here.