Again, I don't know why you are looking at each of these separately. It's a lot easier to be highly skilled and a good skater when you are 5'10 than when you are 6'3 230 lbs.
This is the list of forwards that are 225 lbs or heavier this season
View attachment 815963
How many of these are also good skaters and have top-line talent? Less than 10.
Mantha isn't a unicorn because he's not a top 6 forward anymore. Byfield and PLD are unicorns yes.
If each nhl team has 13 forwards, then that means there are 416 forwards in the league. 10 out of 416 is 2%. So only 2% of players are of the same type as Slafkovsky, which is why these kinds of players are considered unicorns.