Given that there are no good sources of Nintendo storage out there I don't know how profitable it'd be to make a bootleg single-game cart when you could instead sell the same hardware as a flashcart. Used Switch games aren't that expensive anyway. I guess it's technically possible, though.

A false positive is almost weirder, because what does a false positive look like? A false positive on what test? Admittedly I have no idea of how they're ID'ing flashcarts to ban them. What they have clearly works, but without knowing what the technique is I can't tell if a false positive is even possible. The "bought a cart that had been used to make a known dump" theory is... possible, but I'd need more proof than just sounding more plausible than anything else.

Either of those hypotheses shows that their EULA overreach has practical implications that they should have considered, but it's fundamentally different from what the article is putting forward.