What traits are required to effectively select and execute lines at race pace?
Proprioception, reflexes, mental acuity, coordination, agility.
Basically the traits of raw athleticism.
Yes, planning and practice helps, good equipment choices help. Fitness is required to allow you to execute these things over the course of a run.
Some of this comes down to nature vs nurture, but what makes a fast downhiller actually doesn't vary much from what makes a great golfer. 90% genetic lottery. Look at 10 year old Jackson Goldstone.
The trouble for both golf and MTB is that a lot of people are making money off the idea that you can improve by some totally unrealistic amount, when the reality those pro golfers were probably better after one round of golf than the average weekend Warrior can ever hope to be, same for the 10 year old downhillers.
If Joe average rides a course in 2:30 and pros are running it at 1:30, a pro after a couple months on the couch is still probably 1:40, but assuming they have no blatant issues, the average rider might train very hard and not crack 2:00.