How do you suppose that 'eat like a sane person' (as though that were precise advice) gives exactly optimal nutrition, with no gains to be had from any increase or decrease of anything? It seems vanishingly improbable that there is no substance that a human could benefit from getting slightly more or less of in the diet.

It does not seem vanishingly improbable to me. Any organism that must eat complex foodstuffs (which includes all animals big enough to see) must deal with the fact that they have no way to obtain precisely the right quantity and proportions of everything that they need. Their bodies therefore need to be robust to wide variations in their dietary content, resulting in a plateau, possibly a very wide one, between deficits and excesses that do measurable harm. If this is so, then there is no such thing as the exactly optimal amount of a nutrient. Instead, there is a broad range, and if you manage to hit that barn door it doesn't matter where.