The reason people say that--and really, most say he's the best of the Open Era--is because he's got the best statistical record of any non-Slam winner of the Open Era and, at this point, it isn't particularly close. Consider:
2 Tour Finals (only non-Slam winner to win it twice)
1 Olympics
7 Masters (most by non-Slam winners, +2 over Rios)
24 overall titles (5th most by non-Slam winners after Okker 34, Ferrer 27, Clerc and Gottfried 25)
3 GS finals (tied with Ruud for most without winning)
We can say that there were more brilliant non-Slam winners, in terms of peak ability (e.g. Nalbandian, Rios, Mecir), but in terms of career resume, he's top of the pack, ahead of guys like Okker, Gottfried, Ferrer, etc.
Anyhow, to me this makes him an interesting figure, historically speaking. If he won a Slam, he'd sort of be like another Goran Ivanisevic: blocked by better players in his prime, but improbably snuck one out when no one was expecting it. In truth, Zverev's record looks very similar to the better single Slam winners like Ivanisevic, Chang, Roddick, Orantes, Gerulaitis, etc.