My exact first thought. So...what else did he make-up?
With what hope of success would Josephus have perpetuated this fraud? He was writing journalism, not history; there would have been thousands of Jews and other locals who would have heard of his account and said, “Essenes? What on earth is he talking about?”
He would only have immediately discredited himself by trying to perpetuate a fraud a million people could have immediately unveiled.
Now a scholastic fraud 2,000 years later, on the other hand...