It's hard to tell if these people are joking. If they aren't, we have a very serious problem.
Sadly, you can reliably trust that the internet will vomit up harmful pseudoscience all over the place. Thankfully, some people out there who actually know what they're talking about are willing to explain the facts as understood by the real experts.
What makes people want to lie on the internet? Does it give them a weird sense of satisfaction? Do they think they can actually take credit for things that are easy to fact-check? Do they just want to make the world worse? Either way, what feels really nice is to see justice come to them in the form of a dutiful commenter.
Today, history was made, as one of the world's most powerful rockets successfully carried Elon Musk's red roadster (from Tesla, mind you) into a navigable orbit around Mars. Did Elon Musk want to disprove the Flat Earther theory once and for all, or what?
One of the biggest flaws in most conspiracies is the fact that they refuse to apply their HIGHLY specific logical reasoning to all the facts involved.
This is seemingly the case with the Flat Earth Society after their answer to Elon Musk.