Very cool but what does this have to do with "21st-Century Classroom"? Is that just a meaningless phrase to make it sound modern, not old-fashioned?

If you subtract the "meaningless phrase", the title of the book is just "Music Theory". Do this for every book written about music, art, mathematics and you'll just have a lot of books with identical and quite dull titles.

I guess the 21st century classroom is the free online classroom you connect to from your portable device

It's a new presentation, an online hypertext, and uses YouTube publications of 21st Century music as example.

https://musictheory.pugetsound.edu/mt21c/frontmatter-5.html

Could just be because it is the 21st century and this was written for classroom use.