This is a survey of people who joined MENSA. I would posit that most normal, healthy, high-IQ folks never bother, and there's a selection bias at play.
I am normal, healthy, and a Mensa member. What exactly do you mean by that?
Whereas, someone who is lonely, searching community, depressed, etc and _also_ has a high IQ would be more likely to apply and join.
The fact that you happen to be normal, healthy, and a Mensa member isn't particular important when speculating about the possible selection bias. Although, we are just speculating.