While working at MS I remember someone in the office team saying that the original version of Excel online spun to the actual Excel backend and had it output HTML instead of the usual win32 UI. Was I misinformed by chance?
Excel Online was a component of WAC. It was an ASP.NET (and C++???) web application that used OAuth between SharePoint Server and Exchange Server.
So I mean yes, you viewed Excel docs through a webpage just like you do today via ODSP or OneDrive consumer. The backend is completely different in the cloud service, though.
