They're insisting he's going to be shooting at 16% or better at a minimum for his career. Most of the folks who do that and also score at the rates being suggested (30-40 goals per season) are in the Hall of Fame. Therefore, that would be a Hall of Fame worthy performance.
I suppose you could debate whether or not Steve Stamkos deserves to be in the Hall and call it an exaggeration based on that, but that's kind of nitpicky.
Is it possible that he could? Unlikely but possible. I illustrated that he's been consistent all season. The number of people that you are talking about is probably quite tiny and not worth the time.
I'm sure the name Stamkos came up from some weird reason. I don't think even his supporters are suggesting that Karlsson somehow equates to Stamkos. History suggests that this isn't going to last for Karlsson. However you seem determined that he won't reach this total again his his career from what I remember. That is different than a perennial 40 goal scorer. Nash has only reached that 3 or 4 times in his career.
I'll review what they posted that lead you to this, however the posts I've seen before on him have been pretty modest. Could Karlsson be an All-Star at some point in his career? Seems possible. We've had players surprise us from what they were when they were when they were drafted.
WK is far more than people gave him credit for. Our development team, coaching staff, and scouting didn't do a great job evaluating him. Regardless of what next year brings, he's proven that he can be far more than what most of the NHL and it's fans thought. Good for him.