Topics aren't hierarchical but still treated this way

[Cybr]

When bbp_get_topic_post_type() is registered, it has the argument 'hierarchical' set to false.

Yet, in every WP_Query for topics, post_parent is populated with a forum ID.
This meta key is also stored via bbp_update_topic_forum_id().

So, the hierarchical behavior is hacked into Topics and then rerouted to another post type: Forums.

I'm unsure how to interpret this: Are they hierarchical, and should this value be changed? Or aren't they, and should we consider relying on a non-standard meta key?