I don't doubt Slaf's ability to be an NHL caliber player - which means some good games and some bad games.
The title though is specificially for his 1st overall status. For the best player in a draft class, he hasn't been great yet. However, he's also not the worst 1st overall pick ever.
If Slaf was drafted in the 5th round, he would look great right now for his age. It's the lofty expectations of being the best player in an entire draft class that makes him look bad in comparison. Obviously with the 1st overall pick you're expected to pick the best player in the entire draft - so when they're not immediately great - people start questioning them. You don't get those same expectations with later round picks.