I suspect and this ties back to my fan theory that Uhtred is telling a lot of lies to the audience since he's the narrator. I confirmed he was the narrator when I saw that the books were written in first person and directed by him and this also ties into my theory that his cousin rivalry with Aethelred leads him to lie about Aethelred being abusive, incompetent, and disloyal when in reality it was most likely because of Aethelred that Aethelflaed was accepted as lady of the Mercians when Aethelred was incapacitated when he fell ill and even the author Bernard Cornwell himself admitted that he was unfair to Aethelred. Plus in the fifth book, Aethelred prepares for an attack by Haesten that never comes when in real history, the attack did come when Haesten fled Beamfleot and that was known as the battle of Buttington and it's kind of a plot hole why that battle didn't happen as Haesten had to flee Beamfleot after Eadward and Uhtred took it from him but it ties in to my fan theory. So I believe that given the fact that Uhtred is leaving out actual history because he doesn't want Aethelred to appear to have a warrior's reputation, he is also lying about his affair with Aethelflaed, especially when some historians and even Bernard Cornwell estimated that she nearly died giving birth to her only child and thus abstained from sexual relationships altogether, even from her own husband.
I know it was a complicated way of saying it but that's my theory. By the way, you misplaced the N and the U in Cnut, or was that the whole point? That was hilarious!