At the risk of mansplaining, I suspect that actually was what they had in mind, though not exclusively. Friction doesn’t inherently mean PIV. Clitoral stimulation is still down to (the proper amount and application of) friction and (fuckloads of) nerve endings to sense it.

But more importantly, I also think that’s an overly simplistic way of seeing sexual gratification. Sex with a partner is sometimes less directly pleasurable, since the owner of the equipment often (though not always, thanks repression) knows best how to make it feel good, but sex with a partner is still usually better because of the intimacy that comes into play. And sex with a long-term partner can be exceptionally good, because you learn better and better what the other person needs.

(Obviously everyone is different though, and there are always going to be individuals with different experiences and preferences. The world is full of spectrums, not well-defined lines.)