The Champion of Cyrodiil, as in the player-character of Oblivion? You just said yourself that it was in direct relation to the Gray Fox, Nocturnal, and how to break the curse. How is that NOT about the CoC? The CoC is the one who stole it and the one who became the Gray Fox because the curse was broken. That whole storyline could not have happened without the CoC.
It's very debatable that the CoC is the same person as the Gray Fox. For one, if the CoC went off to The Shivering Isles, how did he/she take care of thieves guild? Who would become the next Gray Fox? And anyway, how likely is it that the CoC became guildmaster of all of the guilds in Cyrodiil? And if not all, then which ones
did he become master of?
We just can't assume that they're the same person.