Information Systems (IS) and Data Warehouse
Yes, I'd say that a data warehouse is an information system.  It stores information, using information technology; it's an information system.

That said, I think focusing too much on trying to obtain a precise definition of "Information System" and "Data Warehouse" is not likely to be super-helpful.  Those are just phrases.  "Information system" is a broad phrase that doesn't have an especially precise definition -- it gets used in whatever way the author finds helpful.  And that's OK.  It doesn't need a precise, universally-agreed-upon definition.  It's just an aid to communication.  Ultimately, it just doesn't matter that much how we classify data warehouses; if I told you they are (or aren't) an information system, that wouldn't really change how you should think about them.

My advice would be not to worry too much about terminology and trying to classify things into some taxonomy.  Instead, think about what are the relevant characteristics and properties of the kinds of data warehouse systems you are looking at.  Figure out what is your actual goal, and then identify what characteristics are relevant to that goal, and use that to hone in on the aspects of the system that are relevant.  Not all information systems are alike -- the range of issues that arise in information systems is incredibly broad.