I never applied to Mensa. A cousin of mine (that I barely knew) was in it. He encouraged me to apply on the theory that they had a small scholarship that would help me return to college. He felt my SAT scores would qualify me.
I have seen some discussions on the internet that fit with your hypothesis that many people join Mensa to have their smarts validated. There are other ways to validate that, like getting additional college degrees. These other ways tend to result in real world achievement, like better jobs. CEOs and the like don't seem to spend a lot of time trying to get their smarts validated. They are too busy getting their business model validated.
So, I tend to see Mensa the same way you do: It primarily appeals to people who need validation they are incapable of getting via other avenues, such as pursuing their Master's or PhD.
I have seen some discussions on the internet that fit with your hypothesis that many people join Mensa to have their smarts validated. There are other ways to validate that, like getting additional college degrees. These other ways tend to result in real world achievement, like better jobs. CEOs and the like don't seem to spend a lot of time trying to get their smarts validated. They are too busy getting their business model validated.
So, I tend to see Mensa the same way you do: It primarily appeals to people who need validation they are incapable of getting via other avenues, such as pursuing their Master's or PhD.