In fairness to the author, he is rightly pointing out that maximum efficiency is pretty much irrelevant, certainly in audio-type applications.
He's also rightly pointing out that class-D isn't necessarily as efficient in real-world applications as the marketing/hype would suggest.
In the real world, class-G, especially with the inner transistors in class-B, can be surprisingly efficient and vastly lower distortion than any class-D.
FWIW, various combinations of class-D and switched/pumped/tracking supplies have been around for decades in things like loud-audio radio-pagers.
This article contains an number of errors. The theoretical maximum efficiency of a class A amplifier is 25% for a singled-ended stage, and 50% for a push-pull stage. Similarly, it can be shown that the maximum efficiency of a class B output stage is about 78%.