Having a PNG go from 164.5K to 127.1K as lossless WEBP is not what I'd call "marginal". An improvement of over 20% is huge for lossless compression.

Going from lossless WEBP to lossless JXL is marginal though, and is not worth the big decode performance loss.

Since the person you replied to mentioned MozJPEG, I have to assume they meant that WebP's lossy capabilities were a marginal improvement.