Neither have to come out and say what they do is 'art', because it just is and critics recognise that. When a pop girl starts talking about their work being art, that's when you're in trouble.
Luckily both Bjork and Madonna don't do that because they are secure in their work and themselves. They just create and don't need to explain their creations.
Having said that Bjork is obviously less inclined to consider making her art popular and accessible (after Post) than Madonna, who is the noted master at it. Rolling Stone in their album review of Like A Prayer praised it saying, "
what you hear once you get inside the package is as close to art as pop music gets." They have different objectives in their artistry.
But if by "
authentic" art, a somewhat crude and misleading term, you mean less mainstream, then Bjork definitely wins. But if it's about creating an artistic statement that has huge commercial success (ROL), then Madonna is clearly front runner.
Like I get he wanted to throw in some Gaga shade but did he have to write out such a long and pointless post?