Remember, he's writing a letter to his former self, so the advice applies more to his life than it does in general.
It is always people that don't understand math and its application that say oh, I don't need it for what I do "web development I am looking at you" and then wonder why their systems are, complex, ill performant, and inelegant.
Please learn math before you say you don't need it. Or more specific to this article stating that programming is complex / hard and following that with you don't need math. It's only hard because you are not using math.
I have done 3D simulation, AI, robotics, embedded and now web over the coarse of my career and the only complex systems that I have run into where designed without the assistance of mathmatics.
Of course if you are not competent in math you won't do well at that kind of task, and because you don't know better you won't necessarily recognize your inability for what it is.