remember this jump :P

[Twiggi]

I don't think it qualifies as an exploit in pretty much the same way you say jumps are different than exploits. Exploits are generally considered taking advantage of a bug within the game, something not designed to work in the way that it's being used or something just not working properly. This is simply another 'jump' on terrain, it's reachable by in-game mechanics that are working as intended. The terrain was modeled this way and there's 0% chance that Blizzard doesn't know about this. It's the same as using the inconsistencies in WSG's models to jump up to the top of either base's tunnel.

While I'm not sure how to get up here, from what I've seen its using a form of wall jumping to get to a spot that is not accessible to other players- especially hunters and rogues.
Jumping around the tunnel top is like jumping over rocks. I can still get shadowstepped, I can still get shot at, and its something other players can do or walk around(not that I can ever jump up horde tunnel in combat).

It ends of being a shortcut in a worse case scenario and is not a permanent solution.It does not require multiple jumps and it does not get me in a spot that is 100% unreachable to other players. Terrain jumping is somewhat of an exploit, but not as grossly exploitive as this particular spot. He said himself that it was not really something he intended to use as it felt wrong.