For anyone else who comes here trying to programmatically display the forum index list, you can call shortcodes like this:
echo( bbpress()->shortcodes->display_forum_index() );
For what it’s worth, I don’t know why a function called “list_forums” would display *sub* forums. I would think that would be a function named “list_sub_forums” but, oh well. At least there’s a work around. It’s also worth noting that this might not be the best way to do this. I haven’t done much testing (I’ve done none).
Cheers!
