They're just talking about comb filtering. You get that with a single horizontal center anyways, although two vertical centers will exhibit it at lower frequencies from a single horizontal because the inter-driver distance is different. I don't know why the author doesn't mention that any D'Appolito arrangement (which most center channels use) will exhibit this.
Find any album which has sustained single notes and walk around while listening (early Genesis with Steve Hackett wailing away is good) and you will hear this effect with any horizontal center speaker.
I expect that at lower frequencies the combing would actually not be so annoying but I don't know that for a fact. Interesting point though... but the real lesson is that a single VERTICAL center is perfect except that it normally blocks your TV
