Yeah good old King Combos could only be broke by hitting a certain key combo. Maybe he means something like that
But i just think like stealth here, you don’t really need a combo breaker, as such attacks wouldn’t be much different compared to when you walk up to someone and press 3 times the knife attack key to try and hit them (these combos would just make the transition between moves a bit faster and more spectacular and more variable from gameplay pov and would add different movement which makes it just a bit more interesting, depends on how far you want to go and how it is done). In addition, with current slowdown penalties this would still be a slight disadvantage when you are up against someone with a gun and you try to knife him down.
Oh and i remember: i know the priority opinion wants me to just leave it like that, but i have another idea: a good old ET game “nobody knows”
has that backstab feature, that is: when you sneak up behind someone you can kill him with one knife shot. Of course debatable if such a thing should be added (as i think it is currently not in the game), but if it is added, then i suggest the same mechanic, only that you hit ‘E’ to charge up your knife, to give him a really spectacular finishing blow from behind. Charging up would just be a key mechanic, nothing like “bling bling and now comes the super knife with lighting” … singleplayer games add these options alot, it would have to be underlayed with an anim also. Just wanted to bring that out quick 
EDIT and yes, sry, please don’t take it too extreme again to only neglect the proposal as a whole, because now you don’t actually know what i am proposing, just brainstorming for the most part, it could be a wide field.
=> I would already be satisfied with adding two different knife combos so that you transition between the two knife attacks faster, example: ver.Attack and quickly after that a hor.Attack or hor.Attack and quickly after that a ver.Attack = quicker movement, because of quicker transition <=
I am sure not everyone will do so, so it is just a slight nuance, but worth it and adds a bit of depth imo.