This is a bit of a Red Queen's Race: as the "arbitrary" proper noun becomes associated with property X, people start to respond to it as a generic referent to property X.

This depends on how widely the name is known. "Mensa" is known enough, so many people have associations with it. When a random person on a street will know who "LessWrongians" are, then... well, hopefully at that time the waterline of sanity will be higher than today. But until then, "LessWrongians" means nothing to most people; and if necessary, you can always reframe it as a group of Yudkowsky's fanfic fans.