The Packers had a game against Dak in the regular season to study as well as every other game the rest of the season and Dak still played one of his best games against them in the playoffs.
Nothing Dak did suggests he's a one year wonder. There are things you can look at. When Julius Jones had his really good season, if you paid attention you noticed all his success was coming on the draw play. If you watched Nick Foles one good season, you saw him throwing a TON of interceptable balls and just getting super lucky.
Dak needs more work on his mechanics which will only make him better, but in all the ways that matter, he proved as much as you can prove in one year last year.
The only question mark I have is can he beat Spags and the Giants, because they're the only team last year that actually confused him in both games we played them. As long as he can progress and learn to beat them, they're the biggest test he's going to face.