A. It looks like Safari didn't support webP at the time? https://caniuse.com/webp Either way, they explicitly mention it anyway.
B. If the comparisons keep the pixel size constant, they're not relying on the cool thing about dithering, which is that you can dither down to a small color count and quite small pixel size then display larger with image-rendering: crisp-edges; and it'll still look #aesthetic. From my experiments with the tool you linked, the scaled-up equivalently-sized webPs look potato. This is most relevant for big hero images.
B. If the comparisons keep the pixel size constant, they're not relying on the cool thing about dithering, which is that you can dither down to a small color count and quite small pixel size then display larger with image-rendering: crisp-edges; and it'll still look #aesthetic. From my experiments with the tool you linked, the scaled-up equivalently-sized webPs look potato. This is most relevant for big hero images.