My understanding is that Graves studied Claudius very closely, and virtually everything could have been true. Obviously Claudius’ reliance on the sybil prophecies is an embellishment, as well as this all supposedly being Claudius’ diary. I’ve heard complaints that there is no way that Livia poisoned Augustus (brilliantly played by Brian Blessed).
This was, as I’ve said elsewhere, one of the best things ever to appear on television.