Possibly obvious point, water softeners add salt to the water.

Not all water softeners add salt to the water. nuvoh2o sells a water softener that does not use salt.

While this may be true (have no knowledge | how does it work w/o salts?), the OEM will immediately void your warranty if you use any sort of homeowner water softener, per both Rheem and AOS installation manuals.

I have both; mine are warranted "platinum|10yrs" — why chance it?

and then you heat the salt, which is even better!

Correct. This is the reason.