Im possitive its not DE just becouse at the begining of the book, the PC described that a single missil from us took down the most advanced alien ship like it was nothing, and that was the reason why they didnt disclosure the FTL travel drive, the PC thought humans would wipe out everything and was protecting the other races.
Im with the "they are humans" theory.
What i really wonder is what are those threaders or cables that they describe behind the krell ships, hope at some point we will find out.