if you use an optimal strategy to compress PNGs, WebP lossless is still 26 % more dense

if you take usual PNGs from the wild, WebP lossless is 42-45 % more dense

disclaimer: I'm a co-author of Zopfli and ZopfliPNG, and usually best PNG optimizers are based on Zopfli. I'm also the author of WebP lossless.