I think there is a lot of purely circumstantial stuff working in ban's favor. I am not saying ban isn't insanely powerful of course but overall most of what we have seen so far works because of circumstances. the demon king clearly isn't going at full power, he literally can't. And that is even assuming possessed meliodas has the dK's full power. If the body is actually as strong as the DK then it also has a time limit on the real world which would sort of defeat the purpose of the body. Add to that meliodas weakening the DK. Another factor here is that king ran out of power and can't be of much help either. Its not necessarily the case that ban is stronger than king if we consider all the stuff king has done pretty much on his own. In a single day he easily overpowered 4 commandment mael, he then overpowered the original demon from god knows how far and for a moment even fought zeldris. And of course, its simply not possible for ban to actually compare to the dk. If he did then he wouldn't be able to remain in the human world. we saw the difference between ban and the dk back at purgatory and we know ban is nowhere near from that even if he can manage a hit or two against possessed meliodas.