What does "simulatable" mean in security proofs?
That means you can produce a transcript by yourself (with only public knowledge), which has a sequence of values $C_1',\ldots,C_n'$ such that the distribution of $C_1',\ldots,C_n'$ and that of $C_1,\ldots,C_n$ are computationally (since public key encryption) indistinguishable.