The author admits that this article applies to only a specific flavor of music theory, and says that it only covers a small part of it. The article even mentions at the end it does not cover some of the topics you think are missing.

The article is not a comprehensive syllabus of all elementary music theory, and it does not claim to be. I believe you are being too harsh, as this approach to understanding music feels much more intuitive to some (like the author, and myself), compared to traditional music education.