I think once you know this math, the jump to using matrices is pretty trivial. I can understand not wanting to include it... not that I feel strongly about either direction. I just don't think it's necessary for the scope of this super useful article.
Also describing rotations in euler angles creates more problems than it solves in the long run.
It would have saved me a lot of headaches if quaternion based rotation representation would have been introduced to me as the default, which it is in most 3d engines.
