Why is previous_post_link and next_post_link working outside of the Loop?

“Inside the Loop” essentially means that function relies on data from global variables (such as $post) that are set up when Loop runs. Note that this is not necessarily done by main Loop of query_posts().

In your specific code those variables are filled by $custom_posts->the_post() calls and after that wp_reset_query() gets those values back to initial state.