Basically only two paradigms survived the 90s. The Unix way (via Linux, BSD, Darwin) and the VMS way (via Windows NT). Everything else is either ultra-niche, dying, a mainframe so ancient and terrifying nobody will go near it, or only of marginal historical import.

Bear in mind that when this book was written, the average desktop PC was running MS-DOS, and possibly Windows 3.1 if it was new and powerful enough.

What UNIX is being compared to is the other mainframe OSes of the 1980s, from DEC's VAX/VMS to IBM's OS/360 and OS/400