I mean, Pronman also had Daniel Sprong as the 4th best prospect in hockey at one time
Pronman is obviously good, but he's just one source. I haven't seen any sources proclaim that Hallander's upside is so significant that it warrants this kind of praise. Jesse heard directly that the Penguins were not high on Hallander for whatever reasons. I don't like the appeal to authority arguments because no team is right all of the time, but the Penguins have a pretty strong track record of drafting/developing forwards and deciding which ones to move on from.
It's definitely possible that the Penguins are wrong on Hallander, just like Pronman was wrong on Sprong, of course. But I'm not seeing anything from his scouting reports, production or whatever to make me think that they're wrong with him yet. I haven't seen anything that makes me think Hallander's upside is higher than someone like McCann. That's not a bad player, but that's also not a substantially high upside for a prospect.