And therefore is not licensable under the GPL; because no license is needed at all. Ardour's goal isn't to be "compatible with" the GPL (in the sense of allowing binaries to be built containing both GPL-licensed code and their own code); their goal is to be covered by the GPL license for the purpose of constraining what actions users can take. This requires the code to be copyrighted, because copyright law is the mechanism that allows them to require users to agree to a license.