Do you have a personal vendetta against clustering or something?

I don't know too much about linear algebra but I know I had to learn some when my boss wanted me to do unsupervised classification. Maybe the book just considers it a good application of the material.

What do old books have to do with what should be included in new books?

ML is hyped enough already and kids already don't know enough fundamentals. They're never going to sit in front of a Linear Algebra class again so the appropriate use of their time is to teach them as much linear algebra as possible and not to turn it into a class on today's hype train