The source is trying to atribute all geological folding to being soft sediment deformation.
The short answer, is that in the lab, solid rock has been shown to be foldable.
(As Edge very much knows) The subject of geologic deformation is vast. Much detailed study has been done, to understand the processes. Under force, rock will deform, either ductiley (fold or flow) or brittlely (fracture) depending on the conditions.
Moose