I think he uses cells generated by a walking algorithm and then merged into areas, not navmeshes taken from the bsp file. He wrote "learning map by sampling and mergin nav area with a greedy algorithm" or something like that. That could explain why all areas are rectangular, I have seen no triangular or other polygon types in the videos or pictures. The method is described in the link I gave.
About the details well I think that for a commercial game he gave a lot of details
