Yeah it's about a $150 part for me, if that. I'd be interested to see how much flex is actually there because the video could have been <1mm with how they conducted the test.
I like to do this kind of stuff by tying a (non-stretchable) string between two points, applying load, and measuring how much sag you get in the string. Would definitely be worth doing on your car to see if there is any flex in that area with the solid rear seat panel.