This surprises me. One explanation for the mismatch between my experience with Mensa and Adams' is that local groups vary a lot. Another is that he's making up a bunch of insults based on a cliche.

What I've seen of Mensa is people who seemed socially ordinary (bear in mind, my reference group is sf fandom), but not as intelligent as I hoped. I went to a couple of gatherings-- one had pretty ordinary discussion of Star Trek. Another was basically alright, but had one annoying person who'd been in the group so long that the other members didn't notice how annoying he was-- hardly a problem unique to Mensa.

Kate Jones, President of Kadon Games, is a Mensan and one of the more intelligent people I know. I know one other Mensan I consider intelligent, and there's no reason to think I have a complete list of the Mensans in my social circle.

I was in Mensa for a while-- I hoped it would be useful for networking, but I didn't get any good out of it. The publications were generally underwhelming-- there was a lot of articles which would start with more or less arbitrary definitions for words, and then an effort to build an argument from the definitions. This was in the 80s, and I don't know whether the organization has changed.

Still, if I'd lived in a small town with no access to sf fandom, Mensa might have been a best available choice for me.

These days, I'd say there are a lot of online communities for smart people.

All this being said, I suspect that IQ tests the like select for people with mild ADD (look! another question! no need to stay focused on a project!) and against people who want to do things which are directly connected to their goals.