The way I see it, Markstrom is young and stepped in too early (not his fault, due to circumstances). He also had the misfortune of playing on what is essentially a shit team. But aside from all that, he was inconsistent and he let in too many goals at the start of the game, or the first period generally.
I don't think we can simply point to age - Holtby is similarly 23 but I think you'd be hard pressed to find anyone who doesn't think him to be a far superior goalie, at his age than Markstrom. Yes, there are other intangibles like the teams they play on, etc.
Basically, Markstrom could be very good at some point but to describe him as "dominant" now is out of left field IMO.