A lot of what the guy in the video says, though, is contradicted in
this article by F1 aerodynamicist Willem Toet. Toet makes no mention of using fences to separate flow in a diffuser, and he goes so far as to say vortex generators "of any design really" may improve downforce.
(Also, note the last CFD image--especially where the vortex appears in relation to an L-shaped vane!).