They didn't consult a lawyer when they wrote that. The idea that public domain code that is not copyrightable is incompatible with the GPL is simply not true. sqlite is in the public domain and not copyrightable, for example.
Their decision leans too heavily on what that court decision said. It was actually fairly nuanced.
The court explicitly declined to address whether Thaler could be considered the author because he created the AI, because Thaler originally listed the machine as the sole author in his registration application.
It is clear that works generated entirely by AI without meaningful human contribution are not eligible for copyright protection, though it does not resolve questions regarding AI-assisted works where human input is significant, which is a predominant use case with AI-assisted coding.
Finally, it appears they're unfamiliar with copyright rules around non-copyrightable works that are combined with other creative copyrightable works. The result is almost universally copyrightable. I'm hedging because I haven't thought through the cases exhaustively, but I can't think of a single case where the resulting work would not be copyrightable.
> 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.