Naturally, Claudius is going to sign up to learn winemaking from the god who first taught the oenological arts to mankind -- who would pass up such an opportunity? It's probably his duty as a priest, or something of the kind. It's definitely his duty to spread the word.

While he's at it, he puts Crowley's and Lan Wangji's names down, too. It's been too long since they've done something as a trio, and he feels a bit of fondness for the other Crowley (who wasn't Crowley) who liked his wines natural.