Most big-time junior scorers produce tons of points because of their individual skills and an understanding of the league they're in. They are good to exploit the weaknesses of junior defense schemes. This doesn't mean they will become scoring machines in the NHL. Look at Corey Locke's stats.
I think the poster means Zacha is not the kind of guy that could take over a junior game and score 2 pts per night. But he is a guy when placed with talented and experienced players in the NHL, will be able to bring a lot to the table (physical skills, two-way skills, speed) so that he will be a great NHLer. He will make his team better, but probably wont be a superstar.
There is a couple of players like that. For exemple, a guy like McIlrath was drafted very high by NYR, despite him never dominating juniors. A guy like Fred Gauthier, 1st round by TOR, has never put up 1.00 PPG in the QMJHL, yet projects as a solid 3rd line C like Paul Gaustad, a very good NHLer.