That in itself isn’t an indictment of the website. The tool is built on the assumption of being provided an image generated from Stable Diffusion. If you violate that assumption, it isn’t surprising that the tool fails your “test.”

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”.)

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.)