I just interpreted the lack of Harry thinking of Sirius and Hermione perusing the library as Rowling wanting to keep the book more focused

Which brings us back to the gazillion pages she spent telling us about Ron frenching Lavender and Hermione being jealous over it. If the book was to be really focused, she could have gotten that across and moved on...
Anyway.
All the nitpicking in the world isn't going to change the book as it stands, so... Ho hum.