I think you're missing the point. Every player has their limits. Otherwise every player would be the same level. Even from junior ages as young as 6 or 7 there's always the talented kids and the not so talented kids in the one side. They may try as hard, but some just more natural ability than others. Mortimor was born into a footy family and no doubt learned a lot from his father and uncles since a very young age. But what he learns will only take him so far. At this stage he's done well play first grade, but whether he's a career first grader remains to be seen. We may have seen the best of his ability already.
At that young age the good players are just the players that can either run the fastest or are the biggest (hardest to tackle). Usually the players with dominant personalities are chosen to be halves. Now things like speed and size are usually inherited, but playing halfback is a skill acquired over time.
I don't think there is such a thing as a talent limit or being a 'natural halfback' or whatever. But I could be wrong.