Yeah, this is one of those stupidity litmus tests. Believing that the Earth is flat vs. round does appear at face value to be consequential, as the average person never actually has to care about the curvature of the earth. However, it's not really about the curvature of the earth, it's about critical thinking, listening to evidence, and having a sane and rational worldview.

There is physical evidence that the earth is round, and you can even test it yourself. Flat earthers will run these tests themselves (watch "Behind the Curve") and will still convince themselves that the tests are flawed and the Earth is flat.

And being a flat-earther isn't just a simple difference in belief about the natural world. It comes with the belief that most of society is brainwashed into believing in a round earth, that the governments of the world are all colluding to keep the truth a secret, and that there exists some great truth about the "Firmament" (the dome above the flat earth) that those in power benefit from denying society.

So it's not just some simple difference in belief. Flat earthers think you're a stupid brainwashed rube controlled by a global cartel of godless governments colluding to blind the world to the existence of God by denying the existence of the Firmament and and spreading the Satanic evil of scientific knowledge that suggests that all things in the solar system trend spherical and orbit one another.

Flat earthers hold roughly the same beliefs that the people who imprisoned Galileo, and they would do it again if given the chance because they are overwhelmingly MAGA-types, at least in the USA, where lunatic conspiratorial beliefs are quite common.