I’ll get a little technical for a moment, but I know @tony b is a recovering engineer, so let’s roll with some Engineering approximations and others feel free to ignore. (Once an engineer, always an engineer!).
Radiation decays with distance squared. So, in a Weber, where the distance from the coals on the direct side is, say 6”, and distance on the indirect side is say, an average of 18”, the radiative heat transfer rate is nearly 10 times less on the indirect side vs the direct.
Do the same math for the grates of a 23”, where the main is about 18” from the fire, the middle grate is about 12”, and the sear grate is 6”. For the main grate, the distance to the fire is about 18” on the direct side, and (doing a little Pythagorean thing), about 22” to the middle of the indirect side. Repeat this for each grate. Here are the results:
Main grate: Radiative heat transfer on the indirect side is about 2/3 that on the direct side
Mid grate: indirect side is about 1/2 that of direct
Sear grate: indirect is 1/5 of direct
Comparison between main grate indirect and sear grate direct: about 1/13 of direct
I’ll skip directly to the results for the 32:
Main grate: indirect side is 1/2 of direct
Mid grate: indirect side is 1/3 of direct
Sear grate: indirect side is 1/8 of direct
Another data point comparing indirect side main grate to direct side sear grate (I.e., the 32’s two-zone configuration): 1/16 of direct
Last, take a closer look at Dennis’ video. First, that’s the mid grate of a 23. Per calcs above, radiative heat transfer on indirect side is about 1/2 the direct side. Notice something else? The paper is browning fastest from conductive heat transfer via the grate itself, not radiative, which is why we go to the direct side (unless using conduction via a steel of CI to sear).
What does all of this mean?
The definition of “two-zone” is somewhat subjective, but it starts with a differential in radiative heat transfer between two sides of a grill. Let’s say the magic ratio of indirect:direct is 1:10. The Weber achieves this. A high (indirect)/low (direct) 23 and 32 both do this. The difference, then, is in the convenience and grilling area for the indirect and direct sides. For the 23, you have to remove the food and the main grate to get to the sear grate. For the 32, you simply flip from the main half-grate to the sear grate without moving anything. For a Weber, same thing...flip to the direct side or use the cold grill technique.
A concluding comment: in my subjective opinion, the reason I say the 32 is a true two zone grill and the 23 isn’t comes down to convenience for two zone grilling. With the 32, I can complete a two-zone cook without ever moving any grates and without ever worrying about having enough room. The 32 is a two-zone machine.