You can tend to disagree all you want. That's your prerogative, but it won't change how a uniflow tank works and it won't change the fact that you setting up your tank with the uniflo outlet a 1/2" inboard of the pick up makes it not a real uniflo tank. It will get richer when it uncovers and then act like a non-uniflo tank on muffler pressure.
As much as I hate to put my useless real-life experience up against everyone's thought experiments, what Paul is describing was a relatively well-known and pretty ancient (like, Air Trails in 1952/3 ancient) way of adjusting the time response of engines. In fact, you can build a tank as he describes and it will run at a constant head pressure until the vent uncovers, then lean out from there.
It won't abruptly go richer at that instant, because the "negative" ullage pressure approaches the vent pressure (i.e. getting less "vacuum" in the ullage) as the fuel level drops and will exactly match the external pressure as the point the fuel level gets to the vent.
Even the tank shown on the Nobler plans has the effect to a small degree, staying ~constant feed pressure for about the first 1/3 of the tank, then leaning out from there - because the over/under vents are about 1/3 way across the tank.
It is relatively easy to make this adjustable, by soldering a short 5/32 ID through the inboard side of the tank, leaving enough to put some fuel tubing on, and then make your vent a long piece of 1/8" that slides through. That allows you to adjust the effect to your desires. \
Back in 4/2 days, this was used to speed the engine up for the last few maneuvers, but it is not necessary or desirable for modern engines and mine run constant until the final lean-out.
Brett