Clipping planes in Ardor3D

In Ardor3D the frustum clipping planes are computed in

                com.ardor3d.renderer.onFrameChange()

The normals are calculated by just scaling the camera directions left & up,
like the frustum was a prism, instead a pyramid.

It seems to me that culling -that uses these planes- won´t properly work.

Or I´m wrong?