Everyone, everywhere, already knew that. Scientists and scholars already knew that. The inevitable result of this point of view is solipsism. Nothing is provable, not even one's own existence. OK. So what?

Truth may be a matter of probability but that doesn't make it arbitrary. When we can be 99.9% certain the Earth is more correctly described as "round" than "flat" then some probabilities can be more valid as models for truth than others. We can do experiments to prove the Earth is round. Flat Earthers have often accidentally proven it themselves, despite their priors to the contrary. We can see the curvature of the Earth. We can see ships disappear over the horizon. We can take a plane around the planet. At some point, it becomes true enough, and one has to accept that in some cases consensus reality is sufficient even when imperfect.

No model is real, but some models are useful, and some models are more useful than others. Newtonian physics isn't "real" but it works well enough at some scales. Einstein's relativity is also not "real" but it also works well enough when Newton no longer applies. But only a fool would call either model "disinformation."

And besides, rather than leading people to question their priors and exercise critical thinking, the Flat Earth movement has just bred conspiracy theorists who literally believe the Earth is a flat disk and the moon is a CIA hologram. Let's not give these people credit as if they've done any good for the world.

They might want to rethink their marketing strategy.

Wait, in addition to convincing people that the earth was flat? Or did they intentionally pick something they thought was so absurd that people couldn't also accidentally take their example as true, and instead just learn the lesson about 100% objective proof being impossible?