It's an image similarity search engine, slapping on the tag SD attribution for marketing purposes.
Note that just because an image is similar (to human eyes) doesn't mean that it played a more significant role than a seemingly more dissimilar image. It could even return similar images that SD wasn't trained on at all. Even conditioned on providing an SD-generated image, it fails.
(Something doing what it claims to do, as opposed to naive image similarity, would actually be pretty cool and useful.)
By comparison, a “dog vs. cat” classifier that has 100% accuracy on the dog/cat task will, nonetheless, tell you that a slice of pizza is a dog... or a cat.
(You could possibly interpret the results as “if SD had generated this image, it would have drawn most from these sources in doing so”.)