The salt kills the freeliving stage. The heat gets them to the freeliving stage faster by speeding their development. AND high heat (mid 80sish) kills them salt or not. I don't know what stage high heat kills, but it does.
I've done high heat no salt on one tank and I've done high salt no heat on my other. Ideally, both would be my choice, but I didn't want to salt some of my scaleless fishes in one tank and the other I didn't have a good enough heater (but had fish that could tolerate a tbsp per gallon--that concentration severely damaged my Java fern, though).