IMO the process you describe defeats the purpose of tags for me. Often I want to start my search by tag not be notebook. For example, I have a a notebook for each project. Within those projects I'll often have a note dedicated to the project presentation session, so I'll tag that note as a "presentation". Very often I'll want to see an overview of all my presentations, so I want to search purely by tag and independent of notebooks. The main reason I use tags is specifically for making such links between *different* notebooks, not within one notebook.
Thanks to DTLow I now see that I can select the notebook icon in the sidebar, then select "all notes", then go into the tag filter dropdown, and then finally navigate all my notes by tag. Since I use tags as a primary navigation tool this seems pretty convoluted to me but at least it works. It would be much more obvious if there was simply a "tags" icon in the primary nav sidebar to take one directly to a tag menu, which is in fact the case in the desktop app.
IMO this discrepancy in UI between the desktop app and the iPad app is very misleading as it forces the user to adopt completely different search flows on different devices for no apparent reason. When you train your users to do one thing on your desktop app and then defeat that expectation on your tablet app, that's a recipe for user frustration and very inefficient work flow.