That statement is based on my work for How To Brew, where I titrated malts from several sources, referenced many water reports and discussed those water reports with AJ 10+ years ago to come up with the original nomograph - a linear model for color.
Since then, Thanks to Kai and others brewers experiences, including my own, we have learned that it is not linear, and that it is possible to mis-use the tool and brew alka seltzer or Perrier beers.
The Total Alkalinity range of 200-300 ppm as CaCO3, readily occurs and people have brewed good dark beers with it. Could they brew better dark beers with a lower range? Possibly, sure - it would depend on the total hardness and the recipe grainbill as well. Remember, we are trying to balance a triangle here, AND, then determine if the beer actually tastes good. To say something may not be inappropriate is to say that it may be appropriate in some set of circumstances. I am not God, although I seem to play one on these forums (I Say, I say that's a joke son), and I do not know absolutely where these lines can be drawn. I and my friends have brewed dark beers where a total alkalinity of 300 ppm worked very nicely, so I know it is not impossible.
Finally, I do thank everyone for this discussion, it is definitely food for thought when I will be discussing this book project's progress with the publisher this week at the Craft Brewer's Conference.