"Everything in moderation" is just a tautology. "Moderation" for kale is different from "moderation" for sugar is different from "moderation" for opiates. "Everything in moderation" means "the right amount of everything," which is just begging the question of what the right amount is in the first place.
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.
