The icl_translations table is our directory of posts that are associated as translations of each other and assigns the post type language for each post-id.
However, you still need to have an entry for each post id in this table if you want to SEE these post types when WPML is activated.
The action of setting this post type as "not translatable" simply means that there are no translations to be made but the post type will still be assigned a language info equivalent to the site default language.