I think it's definitely a mix of great talent AND excellent training. It's also being born with the right biomechanics to avoid frequent injuries. I don't know what Hunter's VO2 max is, but it's probably extraordinary. You can raise that a bit with training, but you're born with most of it. As the saying goes, choose your parents well.
It would be very interesting to see what would happen with the top 20 U.S. high school runners if they were ALL coached by Tinman. I believe there are at least 5-10 of them who could match Hunter.
When I was a high school runner, I believed that being the best was simply a matter of training harder than anyone else and being lucky to avoid injury. That's simply not true, and one of my biggest regrets is that I never had the proper training/coaching to reach my full potential. What could I have done with higher mileage and no more than 20% high quality training? Top runners like Hunter and Fisher not only have great potential, but also the training/coaching to maximize it. They are so fortunate.