Great questions, from a fellow constellation!
On the server, Gemini leverages Analysis Services running as a service app in Sharepoint. The Gemini workbooks are stored as workbooks, but as needed the server can upload the embedded Gemini content to the AS server and provide typical services such as refreshing data, or serving result sets to a client app as needed.
So, in a sense, yes the Gemini data is available as an SSAS cube - it's connection string is the URL of the xlsx file - and SSAS clients can query it. We show cool demos of Report Builder doing just that. But any other SSAS client could also do so. However, what they see when they connect is a rather funky "cube." Technically, in this version, it doesn't have many of the regular cube constructs such as hierarchies, and for business scenarios a Gemini workbook will typically only address one business question, or at least a relatively narrow domain of questions, compared to a typically broadly scoped OLAP solution.
However, you have indeed identified an important point. A Gemini workbook is not just a workbook - it is a real BI solution, and benefits from the openness of the MS BI platform. It's a bit like a Sagitttarius, which has the mind and smarts of a human, and the powerful back end of a horse!