It depends on those initial emails and what was agreed upon. Assuming no information was left out and he truly said "The contract was for a comprehensive buyout

It depends on those initial emails and what was agreed upon. Assuming no information was left out and he truly said "The contract was for a comprehensive buyout, signing away all my rights forever, which was exactly the thing I’d told Carl that I never did with my work", it could be presumed that what he was paid for was essentially just a license to use his work for the Java game.