Hey guys! I'm a guy. And I'm here to remind you that Necron scarab conga lines were legal before they weren't. There's too many funky rules interactions for GW to predict, so stuff like this happens. When enough people complain to them, they FAQ it, and then the circle of life rolls on.
For the time being, gun emplacements are totally neutral (no matter who bought them), and can be assaulted (since they're a model). So for right now, the trick works. There's a lot of things that have to go right for it to work, though, and in the meantime a good army will be focusing on winning the game, not pulling off a funny trick.
So, yeah. It's an imperfect game, as always. Not worth a fistfight.