exactly which version of MyBB you are using and do you have any plugins from earlier version of MyBB & other (unreliable) sites ?
if you are using latest version of MyBB & using plugins from this site (and few other reliable sites)
then your forum should be secured enough as you are taking required precautions on regular basis.
It isn't assuming you have strong passwords. More importantly, threats are just that, some power-hungry child with nothing better to do. I wouldn't worry.