Before this, there were anecdotal examples of SD outputting an image with some Shutterstock watermark or very similar to some artist's work, but the prompts also seemed highly specific or were asking for something in that artist's style.
This tool at least lets us start to trace back the average image, and so far it does seem SD is adding something novel to its outputs.
There are many better ways, in the sense that they actually do something like estimate the causal effect of a specific training datapoint, like leave-one-out cross-validation training or surrogates to Shapley value, or using nonparametric models to trace backwards. This is a whole subfield of ML research.* (The primary summary is: "it's hard and the easy approaches don't work." Which why he's not doing any of those but an easy incorrect thing.)
* I'm not entirely sure why anyone cared so much... Research topics can be kinda arbitrary. But in this case, I think there was something of a fad around 2017 that there were going to be 'data marketplaces' where you would be trying to estimate the value of each datapoint to price it. This turned out to not exist as a business model: you either used big public data for free for generic model capabilities, or you had small proprietary data you'd die rather than sell to a competitor.
I think this website is evidence SD is a novel image generator, and rarely creates infringing images (at least in the way we thought of infringing before these kinds of AI).
You're jumping to a conclusion that the data doesn't warrant, I suspect because it's a conclusion that suits you.
They're doing something like a "reverse image search" from an AI generated image over the original dataset and returning a few examples that have a high degree of similarity. There's no guarantee that the images they return are actually the ones combined to create the AI image. In fact their results are dubious - see Saurik's comment:
https://news.ycombinator.com/item?id=34671149
I don't have an opinion regarding the question of whether SD creates art or not. To be honest, as someone who enjoys art or kind-of-art in many forms, it's not important to me (although, of course, I can see why it is important to someone who makes a living from art or AI generated images).
However, this website doesn't add anything except noise to either side of the debate, from what I can see.