Dedicated his entire life.
Ray Ewry, the man who set the world record all the way back in 1904, had polio as a child (meaning he wasn't athletically active as a child) and then as an adolescent, went to college and got a degree in engineering. He didn't become serious about athletics until after.
Here's a picture of Ewry:
While he's in good shape, his physical stature doesn't really resemble that of a "modern athlete reared on Eastern Bloc training methods" and yet, a modern athlete with presumably better genetics (let's be real, African Americans are naturally better leapers than Caucasians) bested the mark by less than 10%, meaning we've only seen very marginal improvement over 110 years, despite every athlete supposedly becoming athletic superheroes because "Eastern Bloc Training." And no, Jones does not outweigh Ewry by 50lb. He's 199lb. Ewry weighed in at 175.
The point that keeps going over your head that I'm trying to make is that human athletic ability is pretty much hard coded and no training method(s) are going to exponentially improve athleticism, which means that your modern superhero athletes wouldn't render past athletes ineffective, like your re ed claim that an 80's basketball player wouldn't be able to get his shot off.
On the other hand, we've seen massive leaps on the skill side on things, everything from off hand dribbling to defensive positioning. This is how a modern player would dominate his counterpart from the past.