Damage avoidance reflexes is different than pain, I think that is what they are going for. If a fish feels things touching it, it simply tries to move away. The more pressure that is exerted on it, the more determined it is to move away. In the case of ich, the fish has the response of trying to rub it off. That does not mean that it is itchy, that is just its response to having something persistantly stuck on it. Back when we would tag fish on the reef for study, they would scrape to remove the tags... and a tag through a fin does not itch, you could describe the sensation as other stuff maybe, but not itching.
I will give you a human example of this.
I shattered my tibia playing football many years ago. The damage to the nerve system when it compounded removed all surface sensation, pain, heat, cold, minor shock, etc. Like in those movies, I can poke myself with a pencil or other sharp object and it doesnt hurt. If I exert pressure however, I can feel that force against myself. Over time, I have learned that even though I could not describe what I feel as pain, very fast, or very strong blows cause me to move away from whatever it is, out of an interest in preserving the integrity of my leg.
I am not a scientist, but I would estimate that fish and other less developed families of animals react the same way. In my case the advanced nervous system was damaged, in a fish it never develops.
Yes fish do "freak out" if you touch or poke them too much, they flop wildly if out of water or if sandwitched between the glass and your hand or net, but these are just attempts to avoid capture, a basic prey response. It is instinctive. I don't know about the wheezing fish hook instance, maybe it is similar to our gag reflex, but involves air because of the gills.
JMO, HTH