Heyo! 
General question:
fetchComponentData and fetchPages don’t seem to return pages that are hidden with the underscore prefix. Is this intentional? And is there another way to retrieve hidden page metadata in code?
I think you stumbled upon a somewhat buried feature here, but yes a leading underscore hides a page or component. Unfortunately there isn’t a way to forcefully include it in the results set
Good to know! I managed to work around it with the code editor → I just pulled all of the data with a data component, and did a .filter() on the results 