In this case, he'd based it on absolutely nothing other than his desire to get Antonin on the Skyrail because he thinks it'd be funny. The funny part is where the route to Ketch Heath swings out over the harbor near the Fog. No one ever said Rodolphus was a good friend, but it's not like Antonin is as drunk as he is, so this may backfire.

"It is a small adventure. Nothing like the old days."