PNG is a good baseline for image sizes before adding any kind of smart compression. Nothing wrong with that, and the comparison would not look much different against JPG (except it would probably look actually worse than the video).
For the web, you might use it when you want alpha channels. WebP is still not supported everywhere.
Regardless, it’s useful as a comparison in the article to show just how good h264 is versus other compression techniques. They are not recommending PNG or anything. WebP would only produce a 20-50% smaller file, not 1000x smaller, so it doesn’t really matter which image format they compare to.
