I always assumed it was something like that. A thought experiment that someone who wasn't "in the know," stumbled upon, latched onto, and created an entire subculture.

As I understood it back in the old times…. The Flat Earth Society was a groups of logical rationalist who got together and for shits and giggles (as well as practicing rational debate) would pick an indefensible argument (eg. the earth is flat) and attempt to use rational arguments to support that indefensible claim.

Basically a rowdy drunk philosophers get together.

Edit: there is no proof of this, just something I read online in the early days of the internet.

but snowballed out of control.

I don't think it "snowballed" out of control or whatever. I think that group is full of trolls that don't believe what they say.

Each one of them is a black hole that eats all of your time while you try to debate them.