This is a silly question but I would like to know the answer. Does a Batrep have to be on Orks?
Yes, members of the WoTW who post batreps about their orks receive teef awards.
If you post about another army that is not orks (even if you are a member of the WoTW) then you will not receive any teef.
The WoTW is about and for orks, thus, only activities that are about your ork army, or orks in general, are all you can earn teef for. Since none of the other races in 40k use teef as currency, it would be pointless to pay space marines or eldar with teef, as they wouldn't be able to spend it anyway. That also includes tacticas, painting guides, modeling, articles, fiction, or any other activities that you can participate in on these forums.
Hope that explains it for you.
PS. I will update the bank account to reflect this answer so that others in the future will know, and knowing is half the battle.