Against Carlsson's peer age, which was the rookie tourney, Carlsson was the dominant forward on the ice... and sometimes on the dominant forward for both teams on the ice.
In pre-season, Carlsson was doing the same thing as he was in the rookie tourney, but against stronger competition and that's when his game got stifled. Maybe Cronin overworked him like the rest of the team with full practices in the AM and a game in the PM of the same day. Big Carl is just 18-years old and will take some time for his body and mind to acclimate to the NHL rigors.
IMO, Carlsson is a dominant player, but isn't the flashiest player. In games where Fantilli faces stiffer competition (WJC, NCAA final four, or WC), he struggled offensively. Yet in games where Carlsson faces stiffer competition (WJC, SHL playoffs, or WC), his game rose or exceeded expectations. If one only did surface research on Fantilli and Carlsson, then one would be all enamored with Fantilli b/c his game on video highlights are viscerally entertaining. If one does a deep dive on both, then you have two high end players that play a different game - one has violent speed and the other has calculating IQ that works with his teammates.