This is with the camera moved, and the walkmesh projected into view space:
Most of the tiles are completely behind or in front of the walkmesh, only a few tiles you can walk both in front of and behind are actually set at an appropriate depth. Most noticeable in this is the room divider. (And in this scene a lot of surfaces are marked in the walkmesh without actually being reachable.)
I was hoping that they have an automated process that calculated the tile depth automatically from a rendered depth map, but it looks like it was mostly manual artist work. Otherwise I could've extracted more depth information from the rendered image.
And this is the opposite, field tiles projected with the inverse of the camera matrix into walkmesh space. (Just got this to work after fixing an absolutely "Doh!"-worthy error.)