Many thanks for a thoughtful comment, Rafał! I fully agree with you: we need to clearly distinguish between cultural (1) phylogenies, (2) genealogies, and (3) social networks.
The question of depicted units isn’t really easy to solve. For example, Fig. 3 can be interpreted as a kind of social network, or, instead, we can stress the fact that every “person” on the tree is just a population of books, each of which can share some features (such as the free indirect style). I do prefer the latter view.
The genealogy of an art form is “unrolling” within a social network of some sort. This network can (and probably should) be taken into account (because, as you’ve said, the similarities between genres or forms can be a result of social network effects: friendships, etc.), but in principle it’s a different thing. Social network is almost a “habitat” for cultural species: technologies, traditions, art forms. And this habitat does have its shape too (describing which wasn’t my intention here).
Also, good to hear that you think that mapping interactions and genealogies is more suitable for the modern cultural evolution than building phylogenies — taking into account the abundance of data on modern culture (often, not properly organised or digitised, but still…). In fact, there are many attempts in contemporary DH to build the social networks of notable people from the past. Not so much — genealogies of forms. However, to me genealogies would be much more interesting…