I think the point he was trying to make is that fans often (seemingly almost always) err on the side of maximizing potential or coming close, so expectations might need to be dialed back, the truth is probably closer to one of Mathews, Marner or Nylander will meet expectations, but it's not all that relevant to the thread to be honest.
Here's the thing about my expectations with Nylander (which are different from my expectations of Marner). When he was drafted, I was very high on him. I saw #1C potential. At that point it was just potential - I knew there would be questions about if he could get bigger and if he could hang playing against men. Then his draft +1 season happened, and he put up record setting SHL numbers for his age - doing so on the worst team in the SHL. That brought my expectations up from "Nylander has #1C potential" to "Nylander is going to be a top 6 player (C/W) in the NHL, with top line potential. Then he tore up the AHL last year, and put up the third highest PPG of all 1st year players (behind only McDavid and Eichel) and my expectations were solidified as "he's going to be a top 6 player in the NHL, and it's probably going to be as a #1C".
To me, that is a reasonable direction to take in terms of expectations. Because Nylander is still yet to do anything to prove otherwise. He's been a top performer in every level he's been in so far, and for a rookie in the NHL he was behind only 2 of the top franchise centers drafted in the last 5 years.
Marner, on the other hand, does still have more uncertainty. I recognize that his ceiling is extremely high (probably higher than Nylander's), but I also don't expect him to reach that. He's got top 5 NHL winger potential (look at his production, he does), but I really don't think he reaches that. When I look at Marner my expectations are "he has elite #1W potential. He should become a top 6 winger for the Leafs, and I'm fairly certain he will, but he's got a lot of uncertainty around him and could fall well short of his ceiling"
Now, if Marner plays in the NHL this year, and proves that his size isn't a concern, and that he can translate his skillset into becoming an effective and productive winger (obviously doing so next season as a bottom 6 player) then I'll have to update my expectations. If he struggles, I'll have to update them as well.
Sure, anything could happen but what evidence is there to suggest that Karlsson would regress by 15-25 points from his career average while Mathews is a guaranteed #1C?
There's really not anything, just like how there's really not anything to suggest that Matthews won't be a franchise #1C. Look at his comparables. He has better numbers than Eichel did at his stage in their development.