18-year-old Bron in literally his first NBA game ever is the counter-argument.
Or this one just 5 months later when he was barely 19 against the reigning EC champs:
20yo Bron averaged 31-7-7 and two steals/game in a slower era on a team full of scrubs where he had 100% of every squad's defensive attention focused on him. 1st-team All-NBA, 2nd in MVP voting, All-Star MVP, averaged a ridiculous 36-8-6 in his first playoff series ever with two game-winners to almost single-handedly defeat the Arenas-Jamison-Butler Wizards, then came one play away from defeating the title contender Pistons.
LeBron as a teenager was already showing better shooting, passing, court vision, leadership skills, and defensive instincts than Zion has ever given any indication of. Not to mention that he was taller, faster, with a more consistent motor and had a more sustainable NBA physique. So Zion's ceiling is higher by....what standard exactly?