Essentially, professing absurd beliefs is a sort of shibboleth to demonstrate in-group membership. The absurdity is the point, because it differentiates them from outsiders. Normal people make true or factual statements all the time, that's not a good differentiator! The statements need to be false and/or absurd to function for their intended purpose.
The Flat Earth mythos isn't even that outlandish compared to mainstream religions, and well within the same wheelhouse as the type of beliefs used to identify the in-group in cults. Scientology comes to mind, but there are many others.
I highly recommend watching the entire video, because there is a lot more to the phenomena than I initially suspected.