I'm going to lunch, but I'm about half way through David's video.
* An inconsistency in his logic is that, an open Mini Cooper header 'HAS' backpressure. It's not 'zero.' David simply has not accounted for it.
* The exhaust gas encounters backpressure at:
1) tulip valves
2) initial mandrel 90-degree bend
3) internal friction within the length of each four primary runners
4) interference where 180-degree out-of-phase primaries siamese
5) where siamese twins combine into collector
6) length of the collector
7) interface with the atmosphere
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8) the 'pressure wave termination boxes', while a 'void' are not at atmospheric pressure, as with the open header.
9) both 'pressure wave collection boxes' DO have bell-mouthed entries, which WOULD kill vena-contracta losses when entering the exit pipe to the rest of the exhaust system.
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10) In 1996, Sport Truck Magazine commented that straight-through glass-pack mufflers actually hurt performance do to internal turbulence caused by tube perforations and spirals, producing more backpressure than the muffler they replace.