Heaven naturally cannot be defied by just anyone. Although it cannot be claimed that Heaven is undefiable, there are certainly very few beings with the power to defy it. In other words, with ordinary abilities, Zhou Tian's Immortal Body should indeed be indecipherable.
The reason why Pan Shi's Divine Power Attack had an effect on Zhou Tian's Human Emperor Immortal Body was simply that both their strengths fundamentally belonged to the same system. When energies of the same nature collide, naturally Pan Shi's Divine Power Attack could pose a certain threat to Zhou Tian's Human Emperor Immortal Body.
However, although they were of the same ability, once Zhou Tian knew the details of Pan Shi's Divine Power Attack, he let Pan Shi attack him without worries.
If Zhou Tian's thoughts were correct, he was entirely confident in saying that no matter how Pan Shi attacked him, it would probably be impossible to pose any threat to him in the end.