It isn't his celebrated craft that grants him authority, it is that he's seen a damn sight more films than most people, knows a lot more about the industry than most people, and has done a lot more for film preservation than most people (without him, we wouldn't have the complete version of Powell and Pressburger's masterpiece The Life and Death of Colonel Blimp, to pick just one example). Therefore, his insights are valuable and it is foolish to dismiss them out of hand, to say the very least.

I do agree that he's not the sole authority, but let's be fair: At no point has he claimed to be the sole authority. He is merely expressing his opinion in a forthright manner. Good for him, because he has several good points.