Earlier quoted context omitted.
From a technical perspective, what are the differences between the two?
The original lossy WebP format is more or less an intra-coded frame format from VP8. This "I-frame-as-image" approach was considered good enough for some time, for example AVIF largely follows the same structure. The WebP lossless format (internally "VP8L") is the second codec supported by libwebp but otherwise independent. This format can be thought as an optimized PNG; it uses the main PNG algorithm but then tweaks…
WebP's select predictor and some other predictors are used in JPEG XL. The same for 2d locality map in backward references.
Both are able to use delta palettes. JPEG XL goes further by having a mixed mode where some entries are deltas for smooth gradients while others are usual palette entries.