That was a different time. Players are peaking much earlier, including defensemen. Look at the impact guys like Heiskanen and Chabot have made at their young age for their teams. Look at how quickly someone like Klingberg or Werenski stepped into the league and produced.
Girard, Cholowski, Dahlin, etc., defensemen don't take longer to hit their stride anymore. There aren't many players hitting their prime at the age of 28 anymore.
lol I thought this for a while too. He was a tank as an 18 year old being drafted into the league and was physically mature as it was. I think that played a part in it.