Yeah, it's a common misconception. What AQ salt does is invigorate the slime coat so that your fish is actually over producing slime. That's usually when people see, they look like spider webs coming off the fins and sometimes the body of the fish, that's them shedding their old slime coat because they started to produce more. So in that regard it's good for keeping external parasites off like Ich and Velvet (although Velvet requires more extensive medications unless caught extremely early) and other things because it basically throws the parasite off.
The other thing it does is balance out electrolytes in your fish, so that can be good for the overall health occasionally but if your fish is otherwise healthy, there is no reason to need that.
What really does it for fin rot is the improvement in water, the daily water change is what get's rid of the rot and starts to promote healing. And to further invigorate the healing process, higher temps to boost the metabolism can help as well as a high quality diet like their natural diet of mosquito larvae, NLS and Omega One pellets and a variety of Frozen foods as well. Staying away from most flake foods, freeze dried foods and other less quality foods will be good. NLS flakes and Omega flakes are still good, or rather better than other brands because of their formula, it's just all high quality.