It's unfortunate that accessibility is so often neglected, but at the same time this tutorial seems to be more about the basic concepts of how a UI library works, not a complete overview of every aspect.

They also didn't include text rendering for scripts that are written right-to-left, nor for scripts that are written vertically, and that affects far more people than those who use screen readers.

Clearly they are not intending to provide a complete and comprehensive course on every aspect of the ideal UI library.