I would not agree it means "the right amount of everything". That would be "everything in the right amount". I take "everything in moderation" to mean not too much of anything, using your best judgement of what too much is. It's subjective and based on intuition, to be sure, but I think everyone has a decent understanding about when they are having too much of something. And I fully admit it's not a great tautology. But in lieu of better science, at least it's something.