I think you're conflating a few different things. Chroma doesn't fill gaps, low resolution chroma channels introduce artifacts of their own.
This is spatial resolution, 10 bit color channels is quantization resolution of the values. Everything contributes to banding artifacts, which are just noticeable changes in values when that are meant to be perceptually smooth, but the luminance channel is the most important, which is why it isn't subsampled.
These are fundamentals of image and video compression and not unique to webp.
This is spatial resolution, 10 bit color channels is quantization resolution of the values. Everything contributes to banding artifacts, which are just noticeable changes in values when that are meant to be perceptually smooth, but the luminance channel is the most important, which is why it isn't subsampled.
These are fundamentals of image and video compression and not unique to webp.