The package manager is an integral part of how the language is intended to be used in everyday practice, so it makes sense that it would be mentioned in the book, though.
And yes, I’ve done this (start a new project in a plane with no internet). Many times. eg: the world’s only PalmOS 5 device emulator (uARM-palm) was started while I was in a plane.
And if you really dislike Cargo, nothing is stopping you from using rustc + make directly.
The package manager just offers a common interface for interop, you can still build without dependencies.
If the packages are already available locally, cargo works offline.