No, he was right. It's not the character that's copyrighted per se -- though it can be a useful shorthand to talk as if it is -- it's the works containing the character that are copyrighted.
Using the character is no different from using any other part of the work that the character appears in.
Also trademarks are inferior to, and no substitute for, copyrights. When the copyright for Steamboat Willy ends, a significant amount of the trademark protection for Mickey Mouse will end with it. This is because the use of a trademark indicates that all such marked goods originate ultimately from a common source. If anyone is allowed to make new works which are derivative of public domain Steamboat Willy, as copyright permits, the MICKEY MOUSE trademark for such works cannot serve to indicate a single source. Therefore the trademark suffers genericide.
The mark can still survive in other contexts -- Peter Pan is both a public domain character in the US and also a trademarked brand of peanut butter and intercity bus services -- but the copyright does some serious damage to it.
Look into the Shredded Wheat case for a similar situation involving the expiration of a patent dragging down a trademark, and the Dastar case for trademarks not being permitted to serve as a substitute for copyrights.