I don't know if it's a flaw per se, but you're not only removing uninteresting people, but also interesting people who aren't interested in taking online quizzes.

> Being able to prescreen is less friction than spending time meeting uninteresting people.

This is probably true for some individuals (e.g. those with social anxiety, or those who work 16 hours a day or something), but certainly not all. Even uninteresting people are often moderately entertaining for a short while, which is much more than I can say for taking an online quiz.

Additionally, there's a bit of a chicken-and-egg problem regarding the reputation of the group itself. If I can be pretty sure ahead of time that being admitted to the group will be worth it, then I will probably suck it up and take the quiz. This pretty much requires someone who is already a member convincing me. Otherwise it might well turn out to be a ghost town, or full of holier-than-thou nerds, or just not a fit for whatever other reason. I wouldn't want to go through a prescreening process just to find that out -- easier and more fun to just stick with meeting randos.