Comprehensive subject?

I was baffled to find this in the introduction to a textbook:

We hope that readers will find this text offering them a useful introduction to and a basic treatment of [...], as well as preparing them for more advanced studies of this exciting and comprehensive subject...

Aren't all subjects comprehensive by definition? Comprehensive with respect to what?

In my opinion, not much thought has been put in to this sentence however it can have some meaning. I believe the meaning the author wants to achieve in this text is something along the lines of "as well as preparing them for more advanced studies of this exciting and extensive subject...".

A subject can't really be comprehensive as comprehensive is sort of like how in-depth something is. For example, to do something comprehensively is to do it thoroughly - covering all aspects in sufficient depth to be comprehensive. The boundary between whether something is comprehensive or not is a grey area and should be judged by someone with authority.

Comprehensive basically means all-inclusive, covering all related information. I think the phrase "comprehensive subject" is an example of poorly chosen word blending. One could deal with a subject in a comprehensive manner by saying all there is to say about it, for example. But by definition a subject would be considered comprehensive in that the subject contains all material related to the subject as part OF the subject. You could not say someone is a "comprehensive person". The term is not being used correctly. Same thing here.