That is speculation. The Bible does indicate that Adam and Eve had numerous progeny but it does not explain why they were all exiled and how they gathered East of Eden to build the city of Nod.
You either have to believe that all of Adam's and Eve's children except the first two, Cain and Abel, left (or were banished) from the Garden of Eden and all gathered together to build a city and live away from God and his gift of the Garden of Eden or that there were other humans who didn't come from Adam and Eve. And that is what Clarence has just realized.