I am really surprised this didn't kick up

So let me see if I got this right, "to stop the spot welds from work hardening you want the body to flex." His assumption is (I guess) that flexure stress relieves the weld. I guess I need to take statics, dynamics and strength of materials over again, I must have missed those dissertations regarding weld work hardening. The one thing that flexure will do over time its cause the weld to fail, guaranteed. Flexure will work harden the weld, make it brittle and cause failure. Classically the weld fails at the weld in the heat affected zone if not the weld nugget itself. And I know that because I have actually run those spot weld failure tests in a lab to test spot weld failure in "U channel" sheet metal, which closely approximates floor pan sheet metal spot welds.
I agree with the rest of you who question the validity of the video.