Because the author explicitly wanted "not to waste his time on banal unrelated issues" except those directly related to the project at hand.

He's not missing important concepts but deferring important concepts to learn other important concepts that lead to progress today.

You end up paying back that deficit one step and one layer of abstraction at a time:

    Some of Brian's biggest forward mental leaps came when he took 
    a step back away from the day-to-day project grind, learned new 
    concepts in a course or from a book, and then thought about how to 
    apply them to SwearBuy.