I took a senior-level undergraduate course in logic that included proof theory and model theory. The textbook was indeed hard to follow and I, too, had to read segments several times, slowly, before I understood all the different things the book author was saying. Parsing unfamiliar symbols was part of the problem, too. I tend to call writing like this "dense", in the sense that it feels like there's a lot more information per word or per symbol than in most other writing. (Sometimes it's hard to tell the difference between writing that's dense and writing that's bullshit, though.)