Look through the lists of players that have produced good numbers in various league and aren't drafted high or aren't viewed as high end prospects. The list is very long, better production in a league doesn't mean you're going to be the better NHL player or prospect.
When you're drafting you aren't only looking at their D-1 production, if that was the case then guys would never rise or fall. All lists would stay the same. You hear question marks all the time about a late birth year players and how much "more" upside do they have, are their numbers due to their added size, strength, physical maturity as they're likely closer to reaching their physical maturity as an adult. If these questions weren't pertinent you wouldn't see players like Couturier, Patrick, Makar being questioned for dominating a league as an older player.
I'm only comparing what Pettersson did in his 17 YO season and what Pastrnak did in his, the fact Pettersson is late birthday doesn't change his production in his draft year. It only changes the way you would view how impressive what Pastrnak did at 17 compared to what Pettersson did at 18.
But they ARENT questioned, not by legit scouts anyway. If they were then Makar wouldn't have gone 4th overall. Matthews wouldn't have gone 1st. Patrick wouldn't have gone 2nd. Tkachuk wouldn't have gone 6th. Pettersson wouldn't have gone 5th. Meier wouldn't have gone 9th. If you assume that their draft year production is "inflated" by their age then why aren't they being "penalized" by NHL clubs?
If it actually IS such an advantage then this would be factored into their draft position and you'd see older players be draft lower than younger players with similar production.
The only people who "question" this are seemingly fans on this board. The actual draft results (both the rankings, the actual picks, and how the players eventually turn out) does not support that being an early or late birthday matters at all.