I think the question is: What level of detail would Michelaelangelo have needed to give the anonymous painter, in this story, for us to still give the creative credit to Michelangelo? If he micro-managed every brushstroke and color, then clearly it was his vision. What if he just said, I want this finger reaching out here and that finger there? Or just, hey paint Adam and throw in God, make it fit this ceiling, maybe tone down that blue a little?
When we say who "made" a movie, outside the movie business, it's usually the director we're talking about, not the producers. They had input, sure, but they didn't control every individual choice.
To keep the sweater analogy rolling, proof of effort is a real thing when you get a gift for someone. If you tell your secretary go buy a sweater for my wife, that's a lot less meaningful than if you go out shopping for it yourself, right?
Presumably, another pope could’ve commissioned the Sistine Chapel’s ceiling just as well as Julius II did, but we assume not many artists of the time could have done what Michelangelo did. So we laud and aggrandize those without whom we believe something could not have happened, especially when, like in his case, they did nearly all the work with their own hands.
Directing a movie is—except in very rare cases—the art of delegation in service of a core vision. And we believe only Stanley Kubrick could (or would) have maniacally orchestrated a space odyssey with effects that still hold up half a century later. Or what have you, I’m not a film buff by any means.