I mean, it's a nice theory but it isn't actually true. There's tons of functionality which exists in many ribbon-ified programs but not actually accessible from the ribbon.

If you right click on the ribbon in many applications and select 'customize' you'll find all sorts of useful functionality. The customize window (at least in Excel 2010, which I happen to be looking at at the moment) has organization to help you find the stuff you can customize the ribbon with, and it even has a 'commands not in the ribbon' section!

Maybe the ribbon contained all functionality when it was first introduced (though I doubt it very much) but that certainly hasn't been true for years.