I do not know that they experience discomfort any more than you know that they do not. So we may just have to leave it at that.
Your simple observations do not say anything more than that you are avoiding obvious problems. When I have a headache, can you tell by looking at me? In fact. most health issues for humans do not show obvious external signals. That's why you get all sorts of other tests when you get a physical exam.
I'm not sure why you have difficulty understanding the issues of putting a freshwater or brackish fish in full strength seawater. Marine fish spend a lot of energy pumping out salt that penetrates their systems. Guppies that are put into brackish water experience cellular changes to try to adapt to the salt and be able to pump it out. This is shown in the scientific literature. Is that forced morphological change comfortable? Are they comfortable when it is completed?
Quite true. There are a few fish that freely move from full salt water to fresh and back again. They are the exception, not the rule. Guppies are not among them. Not sure what you saw, but guppies as a rule do not live in full strength seawater. Do some get swept into the ocean? Sure. Just like tropical marine fish get swept into the Gulf Stream and ultimately into the North Atlantic, to die. Doesn't mean they like it.
Sure it holds weight as an analogy. A fish or person in a box a little bigger than their bodies will not show deformities. McCain was beaten 2-3 times a week for an extended period. That's is obviously a different situation. But I don't want folks do that to their fish either.
You say that as if it is a bad thing, but someone needs to point at that just because you can do something doesn't mean you should.
IMO, pointing out that not everything posted online is a best practice is especially important in a forum like this one for new hobbyists.