I mean, it's certainly possible that he is one of a select handful of people that has mastered a skill that almost no one else in history has been able to.

But given how many musicians and producers exist and how utterly small is the percentage of people who are comparable to Max Martin, I think it's reasonable to speculate that his success might be partially/majorly attributed to factors of luck/survivorship bias/branding/popular-music-as-a-mass-produced-product I mentioned.

There's also a certain amount of positive feedback, I suspect - once you've had one hit, people will want you to write for them and, because it probably sounds hit-like (which stands to reason since your style has already been a hit), it'll stand a good chance of also being a hit which means more people will want you to write for them and rinse, repeat, cash cheques for 20 years, etc.