Against an exciting 60 point player in a Canadian market (although he was a late-round pick with less hype entering the year) and another 60 point winger in a Canadian market.
Market, pre-season/draft hype, team performance, and counting stats all play a factor. I don't think Heiskanen is hurt that much by playing in Dallas, because one, historically market while a factor isn't huge, and two which rookie is excelling in a major market? I guess you could say Van for Pettersson, but its still a west-coast one.
What will hurt Heiskanen is he doesn't have great traditional counting stats, and that he entered the year with a hype, but less than someone like Dahlin.
But, honestly, how many times in the last decade have they really gotten the Calder wrong. Maybe Ekblad over Gaudreau. But, you can't really argue with the Barzal, Matthews, Panarin (unless you ignore injury), and MacKinnon wins. The shortened season had no truly dominate rookies, and Huberdeau won a close year. You could argue RNH over Landeskog, but Landeskog team making such a leap forward got him it. Skinner outscored everyone. Myers put up huge numbers for a rookie defender.
Looking at the Calder historically, most defenders who win, have had trouble replicating their seasons and really didn't have elite careers. The last 3 guys to win are Ekblad, Myers and Barrett Jackman. The last truly elite defender to win it was Brian Leetch in 1989. Berard won it in 97, and it would be interesting to see how he played if he didn't lose vision in one eye.