It seems to me fringe NBA talent proliferates in the NCAA and abroad -- whether or not they're better depends largely on the league. There are pro leagues in Europe that would decimate NCAA teams, and there are international teams that'll take guys who can't make it in the NCAA.
This is an important point.
For what it's worth, Porzingis played this year in the Spanish ACB, which is generally considered second only to the NBA in level of competition.
Folks should also realize that this is professional ball, not interscholastic, "amateur" competition. Some of Porzingis teammates and competitors have been playing professional ball for several years. And like most pro leagues it is dominated by the veterans. It's not a one-and-done culture dominated by hot-shot freshman stars. You'll find far more guys who have physically filled out with muscle mass in the ACB than you will in the NCAA.
None of that guarantees that Porzingis is worth drafting at#17 or not. But I think to suggest that he hasn't faced a similar or even tougher level of competition than the NCAA is a bit off.
I'd also note that I believe that Danny has scouted Porzingis in person so if he DID decide to take him, it would not be a blind pick. He'll have done his homework.