I'm not sure if it was an error in how they originally translated the mod tool manual, but I think the 20,000 tri limit is just a per-layer limitation of the exporter and not so much the engine itself.
So far pretty much all of our models are over 20,000 triangles per mesh. To properly export, you need to split the mesh into different lightwave layers. Each layer can't be above 15k I believe, or else it spazzes out on export.
That doesn't mean you can have a 100,000 triangle mesh or anything, but it does give a little more freedom to make what you want.