This guy either a) did something else to trigger the ban, b) bought a bootleg cart somehow, although that doesn't seem like it'd be particularly profitable to sell on Switch, or c) hit a seriously weird bug. Or, I guess d) is lying about it?

There’s also option E) bought a used cartridge that was ripped. Those pirated cart rips have to come from somewhere, and rippers have no incentive to hold onto the game carts after ripping them. If Nintendo sees multiple identical game carts online at the same time, it knows the cart is a pirate rip and could easily set up an auto-ban for it. Catching the occasional “I bought a used cartridge and suddenly got banned” complaint would be a drop in the bucket for Nintendo. For all we know, this dude was playing a BOTW cart that was previously owned by a person who uploaded it to thousands of users.