I opened the BSP in GCFScape, and it seems all the materials (or a lot of them) used in the map have special VMTs with the "Patch" parameter
https://developer.valvesoftware.com/wiki/Patch

All of them have envmap parameters pointing towards these VTF files which for now just seem like very small black square images that don't serve much purpose. Anyone think this is just another dead end we should ignore?