I'm trying to upload a theme to wp.org directory and it says I need to add all admin pages with add_theme_page(). I understand why and respect it, but we are creating a subpage of custom post types, because WP doesn't have this feature.
Details: We offer custom layouts per each post, taxonomy, author etc.. But since WordPress doesn't generate admin page for custom post type archive/index, we added it, so that user can select unique layout for it, along with title and description.
So, we're only using add_submenu_page for altering layout of custom post types index/archive, which is probably a theme purpose, we think. And it's also only optional, so user needs to manually add theme support for it to use it (no confusion there).