It's about the Soul.Ciel doesn't have a Soul anymore.Yeah,He is alive again but without the soul bc Sebastian used the Soul so that he can come into the human world with a physical form
Ciel is not alive, he's a reanimated zombie since he needs to have a soul with memories to be alive. Remember during the ship arc, people were being brought back to life, but they didn't have any sense of self. All the experimentation the Undertaker was doing to bring reanimate the real Ciel. As Sebastian said, the real Ciel can't be there since he ate his soul, and once someone is dead, they can't be brought back to life. And since he ate Ciel's soul, his soul was destroyed, it doesn't exists anymore.
When Sebastian mentioned in the flashback that there had to be a toll to cross the river into the human world. If real ciel was the toll and was brought back doesn't that means the contract is null and void, since Sebastian can't physically stay?
Or maybe Sebastian lied about that as well, but there was no reason to.
Maybe this just means that the Undertaker didn't secure real ciel soul or the Undertaker was OP enough to return a soul without getting caught.
Maybe even deem real ciel's life worth saving and succeeded through the shimigamis. either by a favor or himself if he was still a shimigami.
So many theories! So many possibilities!!! ε=ε=(ノ≧∇≦)ノ