It wasn't about if King Arthur is actually real or mythical - it is your splitting the difference that you are not at all sure if Mohammed existed or not, but you are absolutely sure that if he DID exist - he didn't molest Aisha!
Similarly I could say that if King Arthur DID exist - he wasn't really the son of Uther Pendragon - that was all made up after the fact.
Do you see how idiotic and ridiculous that sound?
Apparently not!
Remember when they killed Ali? Mo ran out of male heirs so your ordinary rules of inheritance didn't work to justify the people who were in line to become the Calilph.
They made it up. They went on to create the Koran itself out of whole cloth and a Christian missionary's manual on how to convert the Arabs in Mecca.
No one made up King Ad.