That means that SSIMULACRA2 does not capture quality perfectly.
Note that in that figure the formats are compared at the same SSIMULACRA2 score, not at the same file size. In the "very low quality" category, JPEG uses ~0.4 bpp (bits per pixel), while JPEG-XL and AVIF use ~0.13 bpp and ~0.1 bpp, respectively, so JPEG is roughly given 4 times as much space to work with.
In the "med-low quality" category, JPEG-XL and AVIF use around 0.4 bpp, so perhaps you should compare the "very low quality" JPEG with "med-low quality" JPEG-XL and AVIF.
After reading your comment, I assumed you had missed the bpp difference. Please excuse me if I assumed incorrectly.
Note that in that figure the formats are compared at the same SSIMULACRA2 score, not at the same file size. In the "very low quality" category, JPEG uses ~0.4 bpp (bits per pixel), while JPEG-XL and AVIF use ~0.13 bpp and ~0.1 bpp, respectively, so JPEG is roughly given 4 times as much space to work with. In the "med-low quality" category, JPEG-XL and AVIF use around 0.4 bpp, so perhaps you should compare the "very low quality" JPEG with "med-low quality" JPEG-XL and AVIF.
After reading your comment, I assumed you had missed the bpp difference. Please excuse me if I assumed incorrectly.