So, the founder of the group discussed in the OP is my uncle. My dad mentioned to me a few months he was getting into this Flat Earth nonsense, but I had no idea he was *this* far gone.
He has a looooooong history of falling for scams and pushing woo. His whole family is also like this -- the last time I caught up with him he was partnering up with a doctor in South America trying to sell "suppressed Cancer cure" Amygdalin, which was banned in the US because it causes cyanide poisoning:
https://en.wikipedia.org/wiki/Amygdalin
It pains me, given that I'm employed by a university to teach critical thinking.