GNU's healthy now, and will soon be moreso

The only way to distribute a combination of v2+ and v3+ is to abide by v3 (or a later version when one is published). So it's fair to tell people that the project "is under v3". It's simplifying slightly, but not to the extent of being misleading.

To be precise, the project doesn't have a licence. The works (authors' works) have licences.

(If I unilaterally claimed that every part of the project, including the v2+ parts written by others, was v3+, or if I edit all the other authors' copyright notices to say v3+, then yes, either of those acts would be illegal, but that's not the case here.)