Linux was in its infancy back then, being "just another flavor" of Unix with "little to tell it apart" (other than being free, like 386BSD). Linus Torvalds began working on his flavor of Unix because MINIX (a micro variant of Unix, aimed at students) licensing didn't allow one to freely distribute it. He also changed internal programs to GNU alternatives wherever possible. The GNU Project was already rolling since 1983, so it already a significant quantity of software written.

Another thing to keep in mind, the internet was still very small, connection was slow and expensive by the time the book came out. The majority of software distribution happened with physical media, like floppy disks and, later on, CDs. Mailing lists and usenet forums could instantly send messages back and forth, but uploading and downloading stuff was severely limited.

So, Linux didn't immediately kill commercial Unix (MacOS has Unix roots, some big companies might still use Solaris), but it being completely free allowed it to "evolve and proliferate like a virus", much like Unix before it.