Simply by the definition of an exploit, it's not an exploit. Why? Well, Blizzard wants to prevent people from getting an unfair advantage. The fact that everyone can do it means there is no unfair advantage. If you currently own WG and you lose because a bunch of demolishers squeezed thru the doorways...come back in 2.5 hours and do it to them. I'm not saying that it doesn't ruin the WG experience. I'm sure they'll prevent this from happening in the future.

To be an exploit, there needs to fundamental flaw in the design. This is a mistake made by the clipping/collision/size parametrics of the demolisher object. Ever notice how close you need to be to melee a demolisher? You can't hit it if you're standing at the wheels, but you can hit a catapult from a distance. Fixing these parameters will probably break something else (most likely in Strands of the Ancients).

This is an exploit:

Warlocks using their teleport circle to port themselves back up to the top of the Dalaran Arena. Why? Only Warlocks can do this and it prevents melee classes from attacking them. This gives them an unfair advantage.