I'm going to go with what he said a couple times, it depends. I would have been much more influenced if he had shown dyno charts on the same engine with varying pipe sizes instead of just speaking to theory. I fully agree that exhaust velocity is critical. If you check the exhaust diameter on most modern cars they actually get smaller as they get further from the engine, that's to keep exhaust velocity up as the exhaust cools. And his theory on needing large exhausts when you have large valve overlap is kind of against most of the theory I've heard on that. Valve overlap is the period when both intake and exhaust valves are open, and you can actually suck part of the intake charge out the exhaust valve with a system that is designed wrong. In that case I would think you would want a little back pressure. But on a more standard cam profile I would agree, back pressure isn't necessarily a good thing.
I think this is a discussion that will take place as long as we have internal combustion engines.