Should cedar furring strips be used behind cedar siding in a sauna?
Shouldn't the goal for wood longevity also extend to the furring
strips?

I say Yes.
Rip some furring strips from the T&amp;G and your golden.

If not, why not and how long would pine furring strip last under those
conditions?

Unanswerable,  But negated by my answer to - Shouldn't the goal for wood longevity also extend to the furring
strips?