The skepticism is probably because teams don't usually jump from a 60 point basement team to a 95 point playoff team in one off season. Teams also don't usually add the depth of quality of talent in one offseason. They tanked hard last year. Nylander, Marner, Brown, Hyman, and Soshnikov all could've played. And that is a good group to add in one offseason. Two first line forwards, two second line wingers and a third line winger. That replaces Boyes, Paranteau, Matthais, Grabner and Winnik. Thats a big time upgrade. Plus they added a legit top 2-3 dman in Zaitsev.
The game changers are Matthews and Andersen though. Right now they are playing like top players at their position. If they can keep that up, and there isn't any reason to doubt they can, this is a really good team right now.
This roster is a playoff team IMO, the question is whether its this year or not. We've seen ups and downs thanks to inexperience so far. But Babcock is a hell of a coach and the team just keeps improving. If they keep doing what they are doing they aren't going anywhere anytime soon.