Considering that no one today knows whether or not there were other playwrights during his time (that are unknown to posterity) that he might have ripped some of his ideas from, I'd say, at least in terms of debatable discussion, it's possible to claim he's not original, yes.
Posterity remembers those whose works are not lost, not neccesarily the origins of the material.
EDIT - of course, if you can find me the first early-man to ever put pen to paper to write a play/book/draw a painting on a cavewall, I would concede that that could be considered truly original.