Probably not what he intended, but what I gained from his advice is that it's better to deeply understand a project than to just focus on building out features. I often fixate on the latter to the detriment of gaining understanding of the bigger picture. So whereas you could focus on one feature and get it done quicker, if you dive deep and understand more of the bigger system it's part of, you can not only go into features with better conceptual understanding, but you can even understand how changes will relate to other parts of the system and foresee potential consequences.

For any fan of Carmack, Doom (the game), or just video games in general, I recommend reading "Masters of Doom". Carmack's story honestly reads like a programming/general nerd rockstar in the book, and it's a lot of fun. Also helps me understand what kind of personality it takes to be considered one of the best ever at something: for much of his early life, Carmack was essentially awake just to program. His home was basically a mattress and a computer at one point, IIRC.