Hi Michael,
Pardon my jumping in -- I'm not Duke nor am speaking for him, but I might offer a few thoughts.
It's true that the Quads (63 and later models) employ the concentric delay line design in an attempt to approximate a point source. How successful they might be delivering on that objective, I can't say. On paper it makes some sense.
Sound Labs, which I also sell and have owned for about 16 years, take a different approach from many ESLs by driving the entire panel (one piece of mylar with a conductive coating) full range and stretching it over a frame to form vertical segments (strips) to have a focal point (like a lens, from a top view) behind the speaker to approximate a line source. The sound coming off the back of the speaker membrane is not damped; rather, Roger West's distributed bass resonance principle is employed to take advantage of the resonances which naturally occur and reproduce responsive, full bass. This principle is described in detail on the Sound Lab website and on my own site.
You ask about the time for sound to reach the ear from the point on the speaker nearest the ear vs. say the top of the speaker. This may appear to be a concern, however consider that vertical dispersion is negligible, hence much of what is heard is coming from the point on the speaker nearest the ear, i.e. at the same height. No matter where you are relative to the speaker, even in a large room, you are listening in the nearfield. I think Dr. West could best elaborate on why the slight time delay you describe is of little concern in terms of listening. I should also point out that you also get significant contribution from the later arriving reverberant sound field coming off the rear of the speakers, which adds richness without smearing the sound.
I hope this answers your questions reasonably well, but if not perhaps Duke and others will chime in. Another option and maybe the best way to know is go hear a good Sound Lab set up.