One of the tactics of communists is to destabilize a country by ripping out all the foundations of the society. History, morality, traditions all must be marginialized and eliminated as much as possible.
Here comes flat earth, to contribute to that. They love to come up with scientific-sounding explanations why the earth is a flat disk, and ignore basic physics. They toss around a lot of complex sounding explanations while ignoring observable facts and invent their own science. It's all done in an effort to destabilize the country, just like all the sexual madness that's being pushed.
The sun sets.
I've personally spoken on the phone to family in America while I was in Australia, years ago. It was a sunny day for me, and it was dark night for them.
Additionally, I went stargazing in the wilderness while there. The North Star wasn't visible. The night's sky was entirely different.
Neither of those realities are possible with a disk model.