There is no "legal definition" for broken now that we no longer use the DPP characteristics. By your logic, nothing is broken. Knock Off could be considered broken by the fact that it fundamentally changed Pokémon's roles and the meta games. A new toy to play with could be broken too. Gene sect was a new toy; was it not broken in BW? The Mandibuzz example is just terrible. Double Team, Minimize, and etc. are all moves that have been banned under a clause. Is that really different than normal banning?
My logic doesn't say that "nothing" is broken. Just because something "fundamentally changed Pokémon's roles and the meta games" doesn't make it broken, it just means it, well, fundamentally changed Pokémon's roles and the meta games. The removal of permanent weather from Gen 5 to Gen 6 did the same exact thing, but you don't see people clamoring to bring it back. It's not that either was a change for the better or a change for the worse, just that it was a change.
Banning a pokemon from a tier is completely different than banning a move from a tier. Genesect wasn't broken in BW, Genesect was broken in BW OU. It was a perfectly healthy addition in Ubers. Because of the introduction of a new "toy to play with" in Generation 5 in Genesect (the cause), Genesect was banned to Ubers (the effect). Similarly, because of the introduction of a new "toy to play with" in Generation 6 with Knock Off (the cause), certain pokemon moved up tiers, and others moved down. The reason it's impractical to ban a move from a tier (and by extension why it's impractical to call a move broken) is because moves transcend tiering, even if they determine it. It's pretty clear at this point that there are going to be pokes who have knock off in NU, in RU, in UU, in OU, and in Ubers. Some of those pokes might be a tier or two higher than they would be without the introduction of the changes in the move, but that doesn't make it broken, or even overcentralizing.
You have to remember that tiering is based on usage, and usage is based (roughly) on relative viability in the tier. If something that uses Knock Off becomes broken, it will be used extensively in the tier above, or it will be suspected and banned from the original tier. This has nothing to do with Knock Off, merely that the new and improved version of the pokemon doesn't belong in the same tier.
If we banned every new mechanic that a generation shift brought just because it was good, we wouldn't have permanent weather in gen 5, we wouldn't have steel and dark types in gen 2, we wouldn't have fairies in gen 6, etc etc. Knock Off is no different.
Finally, the reason that Double Team and Minimize were banned from the game is because they add an element of randomness and destructive capabilities towards the opponent. It's pretty clear that a move that makes you less accurate is inherently unfun - promoting a game in which good decision-making is prized over random chance has always been a prime objective of Smogon. Secondly, the toxicity that these moves provided to the meta pervaded every single tier, and they didn't really make individual pokemon that much better, with the exception of, like, Moody Smeargle. It was just an annoying move that tanks in every tier could run, not a fairly niche move like Knock Off. Moreover, there's really no unique counterplay to these types of moves. On the other hand, knock off is entirely predictable. Every single time, Knock Off is going to be a Dark Physical move that has a BP of 97.5 if the user is holding an item and a BP of 65 if not, and that knocks off an item. That's not even that unreasonable. Sans the admittedly great secondary effect, as far as physical moves go it's not exactly astounding. The only reason that threats like Crawdaunt are so insanely powerful with it is because it's already coming off a sky-high attack stat with adabtability and STAB.