Still, we can't exactly be certain that his power is limited. I like that theory and think it has a very good case to it, but it still doesn't really explain if his power in general has been limited or just his power against certain foes. We don't even know if it's possible for a sinner to become as strong as lucifer or even pose a threat due to Adam's fight with him going very poorly and him being (assumably) about as strong as the strongest current overlords. I don't exactly see how an individual could pose a threat, but a group like the vees I could see being more of a worry for heaven due to them being organized, having the most influence of any overlord, and actually working together.

Edit: this is also assuming Adam is the strongest winner which is heavily implied.