Well, a wall hack is a cheat where you can see people through walls. Typically, other players are highlighted green/red etc, so that you can track them in the map, beyond what you could typically see.
The same term when applied to your map occurs because people are using tweaked r_subdivisions values, which control how curves are drawn, to improve their framerate. Your map has a lot of curves in it, so when people use tweaked r_subdivisions values, the curves are probably rendered with angles, which probably creates seams between a bevel and the end cap, resulting in the ability for people to look thru a tunnel face, for example, and into the tunnel.
I really wouldn’t worry about. Just realize that curves and tweaked configs result in some odd results, none of which should really beconsidered hacks or cheats.