Seems like he is baking AO straight for the low poly with previously baked normal map as an addition for the baking. Without the highpoly.
I tried this, but mental ray doesn't show my baked normals right. I tried inverting red and/or green channels, but something still looks like inverted. I used xNormal for baking the normal map.
Despite the inverted stuff, baked result is interesting.
Has anyone ever tried this before? At least I've never stumbled upon to this in Polycount.