I agree, diagrams are great -- in their place. But to the author's point, can you draw me a diagram that conveys the same information as the sample sentence from the article:

"Human rights are moral principles or norms that describe certain standards of human behaviour, and are regularly protected as legal rights in national and international law."

I don't think you can, but if you could, it would likely be very large and complex. This speaks to flexibility and information density inherent in text.