That is not entirely true. The basic usage code from the Post 2 Post plugin creates a relationship from post to page in a similar way:
<?php
function my_connection_types() {
p2p_register_connection_type( array(
'name' => 'posts_to_pages',
'from' => 'post',
'to' => 'page'
) );
}
add_action( 'p2p_init', 'my_connection_types' );
?>
But on the admin side I am able to add pages to posts (when editing a post) as wel as add posts to pages (when editing a page), which would make sense for a bi-directional relationship..
