bbp_[get]_reply_post_date( bbp_get_topic_id() );
Really the bbp_reply_post_date() function doesn’t analyze post_type so you can give it anything you want, not only with type “reply”.
P.S. Topics (and everything) is bbPress is a post (class WP_Post), and every post has post_date (https://codex.wordpress.org/images/2/25/WP4.4.2-ERD.png).
I think this way is the most elegant and straightforward.
