As another example, I've spent a ton of time learning game design rules, and all of them are optional, but they're still useful. "Rules" in this context means, yes, communicative terminology, but communicative terminology about particularly effective ways to build commonly understood musical motifs/phrases that affect Western listeners in somewhat predictable ways.
Music theory is a language for talking about a language, music itself. It is partially social convention that leads us to have a 12 tone scale, and it is definitely social convention that leads us to call that a "scale". However, the average Western listener will respond to the notes of that scale in predictable ways, and there are "rules" that you can learn that will allow you to more easily and predictably manipulate that listener's emotions and communicate broader ideas through your music -- many of those underlying rules about tuning, ratios between notes, and so on are derivable from mathematical principles or at least describable in mathematical terms, even if ultimately the reason why listeners respond to some of those ratios is social and arbitrary.
Of course, the rules are not concrete or immutable, they can be broken and often are. But the majority of rules we learn in most subjects are not concrete.
Similarly, there are no concrete rules in writing, and the terminology we use to describe story structure is arbitrary and made up. However, learning the "rules" of writing will make you a better writer for typical audiences that live near you, and those rules are expressed through common set of terms and concepts that many professional/hobby writers have decided to use -- many of which can be partially derived or explained by talking about psychology or history or whatever.
Sure, these are not immutable, scientific principles baked into the heart of the universe, but:
A) the author goes out of her way to say that she isn't claiming that, and
B) breaking down rules and building them back up from different starting points or looking at them mathematically is still a reasonable thing to do with rules that have a social origin, and
C) even though a lot of why certain chords sound good is baked into culture rather than biology, it's also a kind of strong claim to say that all of it is purely social. But I don't think it would matter much even if it was purely social, people who do music theory for a living still talk about math sometimes.