Okay so I briefly forgot that Keon was even on the team for some reason... Ooops! But even so, they're different types of defenders -- I don't think Keon is clearly better than Davion. They just have different skillsets. Davion is way stronger than Keon for one thing, is really hard to move off his spot because he has fantastic lower body strength and balance, and he's also one of the best in the league at closing out on shooters without fouling them or flying by and leaving them wide open. On a team full of guards with hardly any wings, Keon looks like a damned unicorn but that doesn't mean he's twice the defender that Davion is, it's all contextual.
Both guys are pests who apply constant ball pressure to make opposing guards uncomfortable. Where I see Davion as a notch above Keon is that he's way more disciplined about keeping his body between the ball and the basket. Keon is prone to gambling and going for those "poke from behind" steals -- which can result in big plays but also results in the occasional play where he looks a little buffoonish and we give up an easy basket. Davion's weakness is size, obviously, which limits his positional versatility but that wouldn't be hard for a defensive-minded coach to mitigate with sharper defensive rotations and more length at the wing positions.
The idea that Davion's skillset isn't effective in the NBA is crazy though and likely derives from the Kings' preference to only ever put one or two defenders on the court at once. That makes them easy to gameplan around. As long as you can force a switch their effectiveness is mostly nullified. Basketball is a team game on both ends of the floor, not just on offense. Use your players effectively and their relative strengths and weaknesses will cover for each other. Use them ineffectively though and you have a last place team which is equally capable of going on big scoring runs and giving up big scoring runs because they're never in control of the tempo.