We have no idea if that's the case or not. Korn returned during the regular season to work with Holtby, and Holtby mostly continued to suck. Holtby is a notorious head case and the default explanation with him should always be that it's mental. What we know is the team's defensive play had cratered in 2017-2018. Holtby allowed more goals, lost his confidence, allowed even more. In the playoffs, he got to come in after Grubauer had failed and help save the team.
Korn is certainly said to be an incredible goalie coach, and it seems likely that Murray is not as effective as him. But beyond that, I don't see how we can tell much of anything about Murray. There are way too many other factors. Holtby may have just physically lost an edge. He may have needed the stronger defense that was in front of him during his peak years to be at that level consistently. Samsonov has said himself that the season was grueling and he was exhausted by the all-star break. That's something he would have had to work through with any coach.