Dear lauraL,
The author fogot to add the "comments" in the register_post_type() function "options/theme-post-types.php". So I've added this parameter to the custom post types, as you can see now you can comment there "?testimonials=eric-stewart-seattle". But firstly you need to go in its edit page and then select the "Allow comments" option in the Discussion section.
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.