lol wat

This is nothing more than a mere empty threat. Back up your forum just in case and ignore them.

EDIT: As far as you are concerned, the software publisher was the individual whom you directly received it from. They did not personally inform you that they were republishing the software. Because of this, you assumed that any licenses that were packaged with the software belonged to whoever provided the download link that you had used. Little could actually happen to you for this other than your forum getting denied support by MyBB. Just make a backup of your forum, in case your hosting company likes sticking their nose where it does not belong.