Then they are not eternal, boom! An atheist simply doesn't believe ridiculous claims without evidence. I never rejected the idea that the universe is created by some being somehow somewhere. There is no way to know that right now.
If the aliens came into existence by themselves (nobody created them), then this implies that something can spontaneously come into existence by itself. Thus even the universe could come into existence by itself as well, and requires no creator. In such a scenario, Occam's razor would indicate that the universe did not need aliens to create it.
If the aliens were created by someone, we need to find out whether their creator is eternal, and so on until we identify the first cause, or the eternal being that always exist. Thus we would identify him/her as God.
Many theists go back to the first cause from the get-go, they don't bother with intermediary creators, they care about the ultimate creator of everything and who always exists.