I suspect the author of the article is American. Everyone thinks that their own accent is neutral and measures everyone else relative to that. As a non-America

I suspect the author of the article is American. Everyone thinks that their own accent is neutral and measures everyone else relative to that. As a non-American, the mentioned singers don't sound American to me, although I would also agree that they sound neutral. I suspect that a more reasonable explanation is that the phonemes used while singing are more universal than those used in normal speech, and so everyone perceives singing to be closer to their own accent.