What exactly is logically impossible about people with down syndrome being happier on average than those without it?

It could be true because their surrounding/family... would take care better of them than the average person, that I might understand, but still, it's really a stretch.

can you really say you're happy with something when you don't know what life without it looks like? You adapt. You make peace with it. That's human nature. Doesn't mean it's the best option.

It would break the feeling of superiority of people without that disability. Fact. So they can't believe it.

People with legs are objectively superior to people born without legs. This doesn't imply those without have no value but it would be pretty silly to deliberately bring people into the world without legs if you could easily prevent or fix the problem

I'm pretty sure abnormally low levels of neuroticism is a symptom of Downs syndrome. Not a doctor but I've interacted with several. The only neurotypical people who are that happy are Buddhist monks.