Well, it's based on what was on the Internet back then. There's always somebody with something to gain politically from a juicy story that may not be one hundred percent true, and I Claudius, to it's credit, loves a good juicy story.
As Marie Antoinette was waiting to be executed, her captors were making up stories about how she was having sex with her son. Catherine the Great fucked her horse. I once heard a story about how Cleopatra sucked off fifty slaves in one night.