I’ve always understood interaction as a process by which elementary particles exchange gauge bosons with each other or when two masses exert force on each other. For me, the simple act of walking down the high-line consists of many interactions such as the collision of atmospheric particles and my body and the exertion of force on the ground by my feet (exertion of force on my feet by the ground). This definition is concrete, tangible, and most importantly, objective: if there is an exchange of force, there is an interaction.
Thinking of interactivity as a continuous variable with relative measures that outline the “degrees” of interaction is a particularly interesting concept. I do agree with the author that interactivity is not merely a boolean property. Perhaps the existence of interactivity can be defined with a simple yes or not, but there’s definitely much more to interactions than just that. Touching a rock versus playing squash are clearly two different “degrees” of interaction. But the question is, how can we quantify the level of interactivity objectively?