> sqlite is in the public domain and not copyrightable, for example.
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.
Ardour does not seem to require a CLA or copyright assignment, so the maintainers don't own the copyright over contributions either way. The contributor would not be able to sue someone over a GPL violation if their contribution is public domain, but what random small contributor is going to participate in a GPL infringement suit anyway? Doesn't change the maintainer's rights to do so with any of their own human-authored contributions.