It depends how sites are built and how templates are used, but I do think it introduces some confusion: I want to add a “project” but I’m only offered to add a new “page”. You have to know that everything is a page.
Perhaps it is no more confusing than page vs subpage, or the fact that a page is not always a page (ie: an event that only appears in a calendar and never as a standalone page).
In Drupal they solved this problem by calling any content, well… “content” 
But I’m not sure it would work here. Are files “content” then? What about the edit form on the right, isn’t that “content” too?