I wish I could upvote this 200%.
Flat earth is not a theory, it is not driven by an answered hypothesis. Theory in science is a state which is backed up by data, it's not a law but there is information which supports it. Flat earth is more a hypothesis that there is no support for outside delusions, misinterpreted data, misunderstanding of reality, and mislead people who have gotten stuck in a self answering feedback loop of disinformation.
I have this pet peeve too. "Hypothesis" and now even "conjecture" are referred to as "theory". That said, it's just how the English language has evolved, so I accept its usage in daily matters. "I have a theory for why this club's empty today" - that's OK by me.
When it comes to stuff like this, yes, it's an abuse of "Theory" or as you say, even "hypothesis". There should be a new term for "batshit insane misbeliefs that are utterly blind to well established theories with six sigma+ peer-reviewed evidence". Any ideas?
Thanks - I'm guilty of simply lumping those words together too! Clearly the difference needs to be informed :)