It is not like that, check out the last 10 years, there have been very few players who came out of number one overall pick that was as physically as talented as Zion, dude is a beast physically, he is not tall but definitely heavy and still can rebound and dunk very well. Sure, he can still get better by losing a bit weight and playing with a bit different styles instead of just explosiveness but that doesn't mean that he is not good, he is one of the best draft picks of the last 10 years and would totally be in top 20 pick in 5 to 10 years if he stays healthy.
Nobody thinks he is going to be hall of famer right away, or maybe ever, but considering there have been people picked at 13th spot playing better than 1st pick in some of those years, its a good feeling that pelicans picked the clear best of that draft, not "maybe" the best (Ayton going over Luka is a joke for example) but 100% the best in this draft.
Can we really imagine what kind of player Williamson is if he loses some weight? I mean, he's explosive since he was heavy and he would be a monster if he lose weight.
Not losing weight but adding more muscles to his frame will be great for him. I guess he is still young though that's why his body was able to adapt, very agile and athletic.
But if he can turn those excess weight of him and be muscular, he can be more athletic and then physical. That would be scary.