bbPress.org

I say since I think you are trying to set up hiarchal forum and category layout, insert this loop-forums.php file into your child theme instead of what you did.

https://github.com/robkk/bbPress-Hierarchical-Forum-and-Category-Layout/blob/master/loop-forums.php

If you want to try to add the hook into the bbp_list_forums() function, you have to filter the output put the hook before every list item.

bbp_list_forums