I think we're kidding ourselves to think that clustering features of existing works and iteratively removing noise based on that clustering is somehow comparable to building up human experiences and expressing them through art.

Using the "brain in a jar" thought experiment, you're making the assumption that the iterative denoising process is equivalent to the way the "brain in the jar" would generate art. Since the question is whether or not the processes are equivalent, it seems nonsensical to have to assume their equivalence for your argument.