Earlier quoted context omitted.
I understand your point, but that wasn’t my point. And in terms of reality, it’s easy to argue that Matt is the effective owner of the codebase.
The reality is that copyright law treats the author, not maintainer, as owner. Matt is the author of less than 1% of the codebase by line. To relicense, he would need consent from the remaining 99%. I'm not sure how you could argue otherwise.
Yes, if everyone signed a CLA with copyright assignment, as is common in some projects that are still nevertheless GPL, then he could relicense, but because he has not done that (and because it's actually a fork initially), he cannot relicense.