Hmmm. Most states require a red flag or cloth on anything sticking out more than 3’ past your vehicle's rear bumper, like plywood. Has this video been approved by Slate’s legal team?
5' truck bed combined with 8' plywood = less than 3' sticking out past the truck bed (and the rear bumper adds even more cushion). I'm taking your word on the "3 foot" requirement.
In Georgia it is 4 feet. I have never hauled plywood like this. Always with the tailgate down. Cool that it can sit like that without getting interfered with by wheel wells. If it was thinner plywood, say 1/4 inch, perhaps would have bent badly. I hope the do they Maverick 45 degree position with the tailgate.
It’s just good practice to place a red flag or cloth behind the plywood. If it helps avoid an accident it’s worth the extra step. Most lumber yards and even Home Depot will advise to use a flag when hauling overhanging lumber.
In the event overhanging plywood does cause an accident involving property damage, you’re likely going to be at fault if an insurance claim is filed.