Flat earthers are pulling the same trick. They’re right that you don’t know the earth is round. But they’re only right in a context where testimonies of hundreds are disregarded, where widely accepted facts among the scientific community don’t count, where photographic evidence is inadmissible, and so on.
The flat earther’s argument is framed in a context where you can’t set aside the possibility that there’s a pervading global conspiracy – albeit one which somehow intermittently leaves glaring errors which give them away. In that context, you don’t know the earth is round. But in that context, nobody knows much at all and so this conclusion is simply unsurprising.
In the more everyday contexts that we care about, we can rely on testimony. We can rely on the fact that every educated physicist, cartographer and geographer never pauses to think the earth might be flat. And we are correct to rely on these things. If it was incorrect, we’d never get treated at hospitals – for in a context where we can’t trust the established laws of physics, how could we trust the judgements of medical science?