You can do whatever you like with KO, including just using the observables, and not even necessarily in the UI: that's not the point, the point was about design/architectural patterns.
Moreover, the fact that in the docs they say KO=MVVM is a *mistake* and, partly, a marketing stunt, since MVVM was the hit parade of cool few years ago, thanks to the big vendors. But, as far as patterns go, it is *MVC* that is the most general and most fundamental pattern (for "interactivity"), encompassing MVVM, MVP, and all the others, which are rather simplifications/specializations of MVC...
Julio