Flat Earth is more of a sign of protest masquerading as a theory rather than an actual theory. It appears to be more focused on finding elaborate ways to deny other theories rather than seeking truth. It is inelegant, illogical, and cannot be stated without involving a host of conspiracies of low probability. (The fact that one can only adopt it given values/traits/issues that let one seriously believe in those conspiracies may also make it useful for social signaling.)
What I find tragic is that STEM-inclined crowd might have sort of triggered that defensive reaction by supplanting religion directly with natural sciences. Sometimes when you point out something similar to this thread (that physics does not make statements of absolute truth, there are only models of which none can be fully provably correct and complete, etc.) you encounter vicious insistence that the world is literally the way it is described in physics textbooks or Wikipedia—and it will stay that way until we are blessed with a new explanation for how things really are. You can notice how to a religious person this might feel indistinguishable to a competing religion, rather than an orthogonal aspect of worldview (it is completely possible to be both religious and a natural scientist).
Come to think about it, even your comment sounds slightly off to me in this way, primarily due to the mention of “explanatory limits”. I would prefer another term (possibly “predictive limits”). Again, “explanatory limits” makes it sound as if GR or QM (or X or Y) is capable of “explaining” something. Taking a model, just one metaphor that fits how some aspect of apparent reality behaves in response to input or another observation, and using that metaphor to explain how things are in some objective sense (which is what I take “explanatory” means) is, to me, a misuse of the model. The fact that the model works for some of our purposes makes it useful, but does not grant it explanatory powers.
I wish more of us techies resisted the urge to see in natural sciences explanatory powers that they do not possess, and instead branched a bit into philosophy—which can actually provide some grounds for meaningful discussions on topics that are out of scope of natural sciences per se. (As a side-effect, this might also make us more understanding and bearable to talk to for someone on a different bandwagon than ourselves.)