The problem is that flat-earthers aren’t just that. They usually believe in all kinds of other kooky stuff as well, and some of those beliefs pose an active danger to society.
Exactly, the same mindset that takes you to “The entire geophysical establishment is wrong/lying about the shape of the Earth, so I’ll listen to this Youtube crank who says it’s a disc instead” will also lead you to things like “The entire medical establishment is wrong/lying about the effectiveness of masks & vaccines, so I’ll listen to this podcast crank hawking horse dewormer instead.”
To be fair, the medical establishment did lie about it, but not because of some weird “big mask” or “big pharma” conspiracy, but because they have a tangible impact when used by large groups and overselling them would have better outcomes than underselling them.
It’s a classic problem those in power have to deal with: tell the truth and get an underwhelming response, or oversell and get a better response.
Don’t take horse dewormers though, that’s just dumb.
Overselling something that is true is not the same as flat out lying about the efficacy of a random pharmaceutical. Not even in the same neighbourhood.