Yeah I agree you could not possibly put all conjugations of a certain grammar point up, which i’m generally fine with. At this stage, if I understand the base grammar point then the conjugations of it usually naturally make sense.
Once it starts really changing the meaning though, I think it could branch off into its own point, but obviously thats a logistical thing for Bunpro.
Perhaps this textbook just assumed the reader already knew とする, and therefore this was building on that. (its an intermediate text book, so obviously there will be holes in any non-beginner lvl book in what the reader has previously learnt)