The question is, I suspect, how long the fish's kidneys have to cope with a drastic change, and you and I agree, I think, that osmotic shock is not the killer that ammonia is, but it is still worth taking into account: I don't like to subject a new fish to more than .001 salinity difference, though I know they can survive more. When I was newer at the marine thing, I had a topoff accident with the whole tank: lowered the sg several points, and discovered it when I got back from a 3-day trip. I ham-handedly and ill-advisedly corrected it over 2 hours and everything survived, except maybe some bacteria. Even the shrimp and snails came through---no credit to me for forgetting to plug in the autotopoff after servicing it! It was also significant that I was LOWERING the salinity, not raising it, which can be much more stressful. [When you treat in hypo, for instance, you take 48 hours to lower the salinity to 1.009.]
My point being, if you're going to make a mistake, indeed, do make it with the salinity, not with the presence of ammonia, which is just not a good thing to have loose, and which is present in bag water. I've personally had discosoma, bubble coral, sponge, snails, worms, asterinas, aiptasia, and 52 other species of invert survive a fairly definite cycle (ammonia), so yes, life is tenacious.
But when a fish has just been through what fishes go through in collection, distribution, transport, and sales, it's definitely ready for an easy ride, which is really easy for us to give---so why not?