Difference Between a Staging and the Production DWH
I think this is a case of the same word being used to describe two different things. 

The first being a staging environment. As you mention, this is a near copy of the production environment architecture and can be used to test releases that are due to be moved into production or to allow users to view upcoming features before they are released.

The second is a data staging area. Although it is called staging, it could actually be located within the production environment (as has been the case in most companies that I have worked for). This area is used to cleanse and validate data extracted from OLTP systems before being loaded into a data warehouse.

Generally, at least in my work history, I have found that production data may not be copied into the staging environment due to legal or logistical constraints. Mostly I have found that it is because developers/testers should not access sensitive data such as personal details, credit cards etc... but it can be copied into a data staging area in preparation of loading it into the data warehouse.

Over time, as you gain more experience it gets easier to tell which one an individual is referring to - but until then, just remember that live data should not leave the production environment and it is perfectly fine to have a dedicated area within production in which you can process your data.