Artistic diversity is an ambiguous term. I think it's better to say stylistic similarity or lack of stylistic diversity. There's certainly diversity within anime, as in the completely unrealistic doll-face with massive eyes, no nose etc v the more face with a nose, smaller eyes, wrinkles etc. But if you have any interest in drawing and the concepts of shapes, line-drawing and silhouettes the similarities are considerable, and contribute to what the layman would say are indefinable qualities that instantly tell the viewer "this looks like anime".
It's not so much meant to disparage anime, just recognizing that if you showed any of those images to a layman, he or she will easily identify them all as the same art-style, with the same name, coming from the same place, and having serious difficulty determining time period, since it's changed very little.