He's looking at this purely from a self-defense point of view. When asked "is traditional karate good for defense?" if the answer cannot be a resounding yes, as you yourself admit, then then answer is no. The lack of quality control in "traditional karate" is not the fault of people like Mann or Bluming, but rather the fault of karateka themselves. Mann & Bluming approach this argument from a very narrow criteria.
He's listed the criteria for what makes a viable self-defense system in the modern world. For the most part, if karate as a whole cannot provide the answers to those criteria, then he is logical in dismissing. Given that he has listed the criteria, you must either counter-argue the criteria itself, or contradict his conclusions.
The "case-by-case, dojo by dojo" argument does not hold water. Either karate is good for reality based self-defense, or it is not.
The fact of the matter is, that a great deal of karate, as it is practiced, traditionally or not, does not address the needs of modern-day self-defence given the criteria listed by Mann.
Outliers, exceptions, or those schools that only partially meet the criteria, do not prove Mann false.