Co-author of the book here. Thanks for the comment. I agree, it does seem surprising to miss NAT. I opened an issue (which we will address at some point): https://github.com/SystemsApproach/book/issues/75

Mentioned in passing in 8.5.6. Firewalls

https://book.systemsapproach.org/security/systems.html#firew...

the textbook is better, I think that might mention it but I don't recall