Porter is closer to Tatum than he is to Kuzma; like a lot closer. Yes his defense is very bad, but he is a rookie that has only played 900 minutes in the regular season, in most cases those players are going to be very bad when they get thrown into the league, especially on a very competitive team where opponents are going to hunt that matchup. I think his potential on defense, thanks to his length and athleticism, is still fairly high and he should become average, if not above-average, if he can stay healthy. He is already a good rebounder.
I do think pedigree matters as well. This is a guy who probably would have gone #1 in the draft if he had been healthy, and has much more skill than Kuzma. He is three years younger than Kuzma, and already a much better three point shooter, a much better ball handler, and much better at getting to the line. If he continues to improve and stay healthy, in the right situation he could average an efficient 30 ppg. He is that gifted.