Thanks for clarification.
The Wikipedia entry implies they're both riffing on the song, not that the cartoon is riffing on the Keaton film.
I guess I'm surprised if it wasn't coincident given only 6 months between releases!
It's also a little confusing how the Lessig refers to the Keaton film as "Steamboat Bill" when it's "Steamboat Bill, Jr." - the song that appears in both is "Steamboat Bill."
You're right that I read it wrong; I'm still not sure if it's correct though.
edit: Having watched them both, they both of course are steamboat themed. Early on, Bill Jr is referred to once as "Willie", and Bill Jr appears to be a musician while Willie is almost entirely musical (using animals as instruments). Those do feel like references. The contents mostly feel unrelated though.