How do you know he's of the Cyrodiilic varitey? because he claims it he is? Absolute fact, in Daggerfall they had three vampire clans, all of them could hide there nature and live secretly among humanity. How do you know its a retcon it could have been intended as a possible lie to begin with. If you were a Volkihar vampire you'd have every reason to lie about your clans abilities make people believe that only the Cyrodiilic strain could hide its nature and the best way to do that, claim your a member of a different clan.
The story's climix doesn't flow as well if the twist isn't that he's Cyrodiilic vampire. He's lamenting that he couldn't find a Cyrodiilic vampire, and lo and behold he's talking to one.
If it was intended as a lie I'd think they'd include something to hint at that rather than have absolutely nothing to contradict it.