Russ Westbrook is 6'7"-6'8"? Arenas is angling for Stephen A's gig.
But in all seriousness, I think the three things he is really discounting are:
1. Zion's straight line explosion, coupled with his vertical athletic gifts. Not only can he (and will he) reliably blow by NBA 4's and 5's but he has the elite athletic gifts to finish over help side players. Will he just impose his will on a team offensively? Probably not but I doubt he's going to struggling to be a plus offensive player, even with the ball in his hands. The player type Gilbert is speaking to is Rui Hachimura, not Zion Williamson. I get what he is saying....and in Rui's case, that is legitimately the concern, his reliance on 2PT FG's in more traditional big man scoring areas.....but with Zion, I think he just isn't evaluating his skillset correctly.
2. He just casually discounts off-ball or transition scoring like it is impossible to do in the NBA, when some of the NBA's most gifted scorers (Klay, Steph, etc.) are additionally great at these skills. Someone might say "well they are shooters" which is true but they are also not being guarded by less skilled 4 and 5 types at the NBA level. It should actually be
EASIER for someone as gifted as Zion off the ball, in the NBA.....because it is likely he is defended by players who are just universally not used to constantly paying attention to their man, when he is weak side or 2 passes from the ball. Most coaches ingrain that in to help side defenders brains and that coaching technique kind of assumed someone like Zion would never exist. If you sag off of Zion, he's cutting in to the lane too quickly for a big to recover, for an easy entry and layup.....if you don't sag off of him, he can more easily backdoor you for a lob or receive a back screen to get open......or simply just anchor in the corner to provide gravity and spacing that a traditional off ball big, who's not a great shooter, would normally not be able to supply. He literally just completely fucks up all your defensive rotations when he floats around off the ball because he can so effortlessly transition from off ball player to dangerous offensive threat....and that doesn't even consider then how he additionally scrambles your defense when you have to be worried about all of the above and then a coach instead flashes him from the weak side in to P&R screen action.
3. Zion has insane touch. Touch of someone who is a guard. Maybe he has a bit less refined post game for the NBA but he is able to convert opportunities at a much higher rate than a traditional freakish athlete....because he isn't just shot putting the ball up. That should allow him to skill refine (assuming he has the necessary work ethic), while still being a productive player at each scoring level.
The space and openness of the NBA game is perfectly suited for a player like Zion to succeed, where maybe 10 years ago you could believe more of what Arenas is saying. Now? I'd bet a lot of money his take on transition and off ball scoring for Zion will be laughably bad.