So do you agree with Gentry's views or not, minor error aside?
He's spot on. No problems there.
There are basically three wrong theories about how wings work. One says that they deflect the approaching airstream down, just as the bucket on a pelton wheel deflect the water jet. That's wrong because it can't explain what happens above the wing, and it's dead simple to demonstrate that what happens above matters more (roughly twice as much) than what happens below.
So wrong theory number two pops up, which is the old "air has further to travel over the top" claim. It's wrong because once two adjacent particles of air have split up acrimoniously at the leading edge, one to go over and one to go under, there is absolutely no reason why they should ever want to get together again, unless they look each other up years later on Facebook and wreck new relationships with an illicit affair.
Wrong theory number three says that Bernouilli's theorem can't explain lift. Well, Bernouilli's theorem doesn't try to explain lift. It gives a very simple and very reliable relationship between fluid velocity and pressure ina smooth (non-turbulent) flow, and if you apply it to the flow round a wing it gives a very accurate prediction of the lift. It's not in the business of saying "why", though, just "how much". Most people who can't get Bernouilli to work don't realise that you need to take momentum transfer into account as well - possibly because they are trying too hard to avoid Error 1.
Forces on fluids arise in two main ways, not counting viscous effects. You can either change the pressure of the fluid or you can change its momentum (by changing its speed or direction). At the surface of a wing, it's pressure that matters, because that's the only thing the wing "feels". A long way (typically 10x chord or further) from the centreline the pressure variations have faded to nothing and only the disturbance in the flow (= momentum change) remains. In between it's a mixture: around a circle of about 1 chord radius from the centreline half the lift force is being transmitted by pressure and half by momentum change. The Bernouilli-is-wrong-boys almost always ignore the momentum changes.
"Why" is a much harder question and, I would argue, not a scientific one. If you put a solid object in the path of a moving viscous fluid, there will be a force on the object. Any component along the direction of undisturbed flow we call "drag" and any at right angles we call "lift". Things which produce a lot of lift are called "aerofoils", but almost any non-symmetrical obstacle will produce some lift, so asking "why" can get a bit metaphysical. "Some shapes are just better than others" suits me fine as a response.
Finally we know that a side effect of producing lift (or a cause - who cares - they always come together) is circulation: flow around the wing superimposed on flow past the wing. Lanchester worked this out in the 1890s but nobody really believed him for another 40 years. It's an incredibly simple relationship: lift = density x free stream velocity x vortex strength. That's how the Flettner Rotor Ships worked: instead of using an aerofoil wing to make the air circulate, Flettner just spun a big vertical cylinder. Viscosity dragged the air round, producing circulation and therefore lift.
Sorry, that's a bit long. Summary: if you want a wing or a sail to work well, try to keep the longer curved side as smooth as possible.