I assume this part of the tips and tricks is aimed at aerial photography. I think with Zenithal they mean photos with the camera pointing straight down to the ground, and with Obliques they mean cameras at an angle.
So the photos with the camera at an angle are not very useful compared to the photos with the camera pointing straight down.
This is for doing a reconstruction of a (more or less) flat terrain. If you are doing a 3D object or a human scan then the situation is of course different.