To me, the point of the model is to hold the business logic of the data. That includes getting the data from the database, and then ensuring that any application-specific processing is done there also. Like if you need to ensure that post titles are always ucwords you could enforce that in the model. (Yes, that's a fairly silly example...)
The model allows all data to be treated the same, no matter where you use it at. If you use it from a library or controller, the business rules/logic are enforced. All in a single place that helps remove the possibility for error.
It sounds like you are using a library to do that exact same thing. So, in your case, I believe it's a matter of terminology. Your library is what I would use a model for. I think model is the more traditional place for it.
To help with testing your applications, it is definitely better to keep your Controller functioning as just a little guy that sits around directing traffic and keep it as "thin" as possible.
