It means all of their job is concentrated in a few hours per year, so they have to be damn good at it. In order to do that, they need training, which is most of their working time. For most other jobs, there is much less time to train, and it doesn't matter as much because what counts is the average performance, not just a few key moments.

For programming, the parallel would be competitive programming. A competitive programmer will spend days studying algorithms like no one else, because it will matter for the hour or two of the competition. For typical programmers the loss of productivity for not knowing the algorithms is less than the time spent studying them.

I don't think they meant to say that NFL players loafs around all year except for a cumulative 17 hours.