The author's point is deeply stupid. As he admits:

> WebP re-encoding of an already lossy compressed JPEG

So... all this shows nothing. Is webp worse than jpeg? Not addressed. He re-encoded jpeg to webp and it somehow didn't magically cure the compression artifacts he's seeing! Who coulda thunk!

Any comparison starts with taking the originals, encoding to jpeg and webp, and comparing that. Or he could repeatedly encode original -> jpeg -> jpeg and compare that to what he has, which is original -> jpeg -> webp