I'm still scratching my head on that one. The mango trees on my other lot receive zero nitrogen (and are on N-deficient soil), yet they have nice green leaves -- because keep the pH slightly acid and provide them with micro nutrients. Per my experience, mango trees will generally have green leaves if they are receiving proper P-K-Ca and micro nutrients. They don't need supplemental nitrogen, but they do want other nutrients, esp zinc, iron, etc.
I can't argue with his results, but not every mango tree responds well to a set-it-and-forget-it approach. I've seen many trees decline and die in that environment. Indeed, some will survive but not all.
I like Dr Richard Campbell and always enjoy watching his videos and listening to his lectures, but I consider him to be somewhat of an eccentric and take his info with a grain of salt.
very interesting point about the desired lack of nitrogen and also the desired lighter green leaves and it's positive effects on the brix index.