The seat back shouldn't be taking any forward strain during the collision. Stock or aftermarket harnesses should not mount to the seat (**unless designed this way by the manufacturer**).
Looking at the broken seats, it's apparent that the seats were taking the strain from the webbing--the holes in the headrest are pass-throughs for the harness and ideally it shouldn't even touch the seat on the way through, let alone anyplace else.
The job of the seat is to position the occupant. The job of the harness is to keep the occupant in position/distribute forces through the occupant (cornering, acceleration, bumps as well as impacts). The seat is positioned by its own mounts and if the occupant is moving enough to cause damage to the seat on forward forces then the harness is not working correctly.
Now, if the damage was done when the occupant was returning to the seat after moving forward (I didn't watch the video due to bandwidth issues currently), the seat may be partly to fault--but the harness wasn't doing that great a job of keeping the occupant in it.

The seats may be junk but a properly mounted and worn harness is critical to accident survival. In all those cases the harness was added post-manufacture and the installation is extremely relevant--perhaps more relevant--than the seat design.