Flat earthers aren't necessarily shills. Flat Earth was originally contrived as a sort of satire work in academia 'back in the day' and has since taken on a life of its own. It's the Kwanza of physics.
Disinformation agents, of course, will use any popular 'truth' movement to seed it with even more lies. The goal is to get people to stop questioning and believe they have found the truth. Obviously, for quite a few people, it was sufficient to incorporate religious beliefs.
Why it is imperative to Satan that the world be round rather than flat… Hasn't really been explained in any satisfactory manner, but, again, these people have stopped questioning what is 'known' and have, instead, created their own dogma.
While I often find the questions and challenges of the FE types interesting and intriguing to seek answers to, the problem I have with them is that it isn't about the question - it is about their answer being the only valid solution, regardless of it being right or wrong (such as not understanding how Newton's laws of motion work and apply to rocket exhaust).
It's also a distraction from what we do here… Not that I can point too many fingers.