Luka does not have a higher ceilling than 'Bron, and it is not just a matter of physical attributes. Consider the level of basketball Luka has played at for the last, say, 3 years. He came from the second best league in the world into the NBA and looks like maybe the most polished and advanced 20 year old we've ever seen. LeBron came in from High School. Like, it is absolutely a fair assessment that Luka is more advanced as a player in his sophomore season than 'Bron was, but he's also played grown up basketball since he was like 16 years old. That shit fucking matters. The experience gap is enormous.
This isn't to say that Luka isn't an outright genius, of course. The fact that the game comes as easy to him as it does is a testament to that. But I think it's worth noting just how much more legitimate professional basketball experience he has compared to your average rookies and sophomores, especially compared to someone who came straight out of High School.