Most people do not want to have visible compression artefacts on the images they put on their web pages. JXL starts from this premise and tries to answer the question: "How small can we then make the image?"
Care must be taken when trying to compare the performance of image codecs by increasing compression density until there are very visible compression artefacts and then evaluating whether A's or B's artefacts look worse: If both A's and B's artefacts are so bad that one would not want to put such an image on one's website, such an experiment gives no insight on what one would pick for images that one would actually put on one's website.
Figuratively speaking, if I buy a shirt, my main criterion is that it looks good in good condition, and not that it still looks good if I put a coffee stain on it.
So, before comparing codec quality at compression levels where artefacts show, always ask yourself: At that level of visual quality, would I actually want to put either of the two options on my website? Now, it is of course tempting to compare "away from the actual operating point", because it is just so much easier to do comparisons if there are very visible differences. Comparing near-identical images for quality is hard. Doing this over and over again in a human rater experiment is exhausting. But that's then answering the actual performance questions that need to be answered.