I don’t know how WP handles these, but the links are not listed in the global $wp_post_types. ACF uses get_post_types() for the location options. It also appears that after a quick look that WP handles these in a way that’s completely different that other post types, if they are a post type. I’m going to say that they are not a post type because they are stored in their own DB table, and not in the wp_posts table.
This has been disabled for a long time in WP. You should probably look what you want in some other way if you want to add custom fields using ACF.
