Here's what I think it is with regard to the seculars who have embraced flat earth.
I don't think transgenderism has anything to do with it.
It has everything to do with a kind of awakening even among seculars that we are and have been lied to by our authorities. When the 9/11 conspiracy came to light, and the faked US moon landing, the lies and coverups about vaccinations and diseases, government experiments on human beings, etc. ... people began to start questioning everything they were taught and told ... and no longer simply accepting it as true because they were told so. So it's related to this new broadening legitimization of conspiracy theory in general. I for one would not put it past NASA and the other space agencies to be perpetrating fraud. In particular, a number of black people have embraced flat earth (such as the basketball player Kyrie Irving ... who used to play here in Cleveland) ... because they don't trust the man (aka the white man) to be telling them the truth.