I think that may have been one massively clutch and high profile defensive play that led to his defensive game being oversold.
In reality, no one really comes into the league as a defensive stalwart right away. Especially not players who were drafted early for their offensive potential. McTavish is still young and has plenty of time to learn how to balance out his game. He cares about winning, I think he'll get there.
I don't recall anyone saying Mac will be like Kesler defensively, especially when the guy drafted in front of him plays defense like Kelser in Matty Beniers. Beniers is the faster skater and possesses a shutdown defensive acumen, but is still trying to show off some offensive skills.
The CHL isn't known for playing defense and the transition will be steeper than someone coming from the NCAA or Europe. Leo is better defensively than Mac right now b/c the SHL rarely allows odd-man rushes in games from all teams, that's how their game is taught and played across the pond.
Year 1 in NHL, get introduced to the NHL.
Year 2 in the NHL, improve offense.
Year 3 in the NHL, improve defense. TBD