There is a whole paragraph in the preface about why he titled it “21st-Century Classroom”. The first sentence contains one very good reason: it’s a full year college music theory textbook with an open license.

The other reasons cited are:

“This text differs from other music theory textbooks by focusing less on four–part (SATB) voiceleading and more on relating harmony to the phrase. Also, in traditional music theory textbooks, there is little emphasis on motivic analysis and analysis of melodic units smaller than the phrase. In my opinion, this led to students having difficulty with creating melodies, since the training they are given is typically to write a “melody” in quarter notes in the soprano voice of part writing exercises. When the assignments in those texts ask students to do more than this, the majority of the students struggle to create a melody with continuity and with appropriate placement of harmonies within a phrase because the text had not prepared them to do so.”

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

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.