What does fiber have to do with protein intake? They're essentially unrelated, besides fiber and protein both being high satiety (fiber being essentially infinite satiety to calories).

Nothing, but there is no RDA for fiber, and the FDA also doesn't say that people shouldn't have too much sugar/refined starch, which they really should.

Better retention of protein so also a better resorption.

What do you mean? Pretty much all calories are readily absorbed outside of extreme levels of excess.