What's even funnier is that I've actually read a math textbook that quite literally did this exact thing.
90% of French technical books assume that you are an expert on the topic.
I once read some portion of a pithy textbook on Representation Theory by JP Serre. While it claimed to be an introductory book, it had the main theorem one would learn in a semester-long course on page 7, IIRC.
Textbooks should really be tested on a diverse audience with N>10, and then the feedback should be incorporated back into the text, and then another N>10 test should be done if necessary.
why? Is this because computer science text book expect you know all mathematics?