Yeah that's the same deal as r_drawothermodels, crudely forcing the GPU to render objects that are behind other objects would give you a partial wallhack, but clientside occlusion culling was still in effect so once it determined that an object was definitely no longer visible it would skip drawing it altogether, and you would stop seeing it through walls. "True" wallhacks were able to override the clients occlusion culling and reveal everything, so to defend against that the occlusion culling needs to happen on the server, which came much later.