Cardio of course is short for "cardiovascular system," which consists of a whole lot of moving parts. Saunas improve some parts of it but certainly not all of it.

Will fixing up your radiator fix your car? Maybe, if the radiator was the problem, but there's a lot of other stuff inside a car to work on, too.

Your body evolved under the expectation that it would be stressed in numerous different ways, but those stressors can all be avoided in the modern era. If you want to most reliably recreate those stresses you need to do cardio and resistance training.

A light treadmill session won't do much to improve your cardiovascular system health either. I mean it's better than nothing but don't expect too much.

Without exercise, you won't burn ATP and thus won't increase mitochondrial count.