Flat-earth theory is pseudoscience: It uses the language of science and pretends to be science, but it makes the fundamental error of starting from its conclusion and rejecting any information or data that does not confirm its conclusion. A scientist tries to DISprove their hypotheses and rejects any hypothesis that fails the tests. A pseudoscientist cherry-picks data that supports their hypothesis and makes excuses for data that does not. Pseudoscientists often end up as conspiracy theorists, claiming that some group is nefariously concealing the truth, when their hypotheses fail.
Flat-earthers are huge conspiracy theorists, claiming that all the world's governments and all the world's space agencies and all the world's scientists and all the world's astronauts are in an enormous cabal to hide the truth. Any rational person would look at flat-earthers and conclude that what FE really is, is a joke that has gone viral.