Exploit in every game is when a player avoids a certain mechanic with a specific activity. The activity is allowed via game itself but it's when a mechanic is bugged with the said activity that otherwise could've happened, there you have an exploit. It also applies for mechanic changes and unwanted things that also shouldnt happen like bombs making bridges platforms instead of destroying them
Clever use of game mechanics in the other hand is when players combine what they have to overcome things that are already in game , just easier. Like los pulls that everyone does in m+. Or kitty squad
There is situations where you cant tell if its intended to work this way or not which developers decide if its exploit or smart play. Like archi tank sacrifice, we dont know if it was intended for the shard to just go away or now. But it didn't break and they were spawned again in the fight so yeah ...
But when you actually break something that is supposed to happen, and it doesnt, i mean come on that breath is class teaching level of example for cheating!
Now you tell me, stacking adds somewhere so you can aoe easier and w/o void zones is smart play or exploit? Blizzard can come up and ban Danish guys but by game design definition its not an exploit since every mechanics IS happening...