A potential KHL victory is not predicated solely on the play of goaltenders. Finland won the last World Championships with essentially a KHL level roster defeating NHL stacked teams several times.
The NHL is only 2 and 1 against KHL clubs in exhibitions.
Cohesiveness, structure and gameplan is just as important.
That said, an NHL club would almost certainly win a series. But I dont think it woukd be a blowout by any means.
The slang term "stacked team" typically means that the team is comprised of all the best players (in this case of the participating nation) available. As in to "stack the deck", or to manipulate in order to achieve an advantage or desired result.
Every nation iced the best possible team with the best possible players that were available, even Finland in that regard.
I honestly think that if the WHC wasn't happening in May during the NHL playoffs every year, the results would have a good chance to be different though.
I do agree with your main point that great goaltending is not enough. Finland played great hockey and fully deserved the Gold medal they won, without a doubt.