Why don't you just skip all these tricks and use hardware occlusion queries?
My guess would be that they're using Blitz3D, where hardware occlusion queries are not available. On a more general note, hardware occlusion queries are useless in my opinion. They're asynchronous, and in order to make them as fast as more traditional occlusion methods like portals, you have to optimize them an awful lot, and even if you manage that, it still won't provide any performance improvement over the traditional methods for most people. Of course, if you're the kind of person who thinks that FPS is the only genre that has existed or ever will exist, then you might well notice some benefit, assuming you can optimize them sufficiently.