I think it's worth considering though how much of that is pure problem solving ability, and how much is enthusiasm. I'd wager that the majority of academic programs could be successfully completed as long as long-lasting enthusiasm and time management are in place. Also worth considering is just how many factors can lead to a negative outcome.
I'd bet that physics PHDs get there with persistence, enthusiasm, and money. Intelligence barriers may not stand in there way but I don't think they're the determining factor.
Yes. You need human-level intelligence. The rest is personality characteristics (like slogging away for years), circumstances etc.
But this could be said for all intellectual achievement. I would go further, and say there are no geniuses, only acts of genius. Perhaps one person "could have done" it quicker than another... but the important achievement is that someone actually did do it.
I'd bet that physics PHDs get there with persistence, enthusiasm, and money. Intelligence barriers may not stand in there way but I don't think they're the determining factor.