Note that the concepts of luma noise and chroma noise are not rooted to the physical phenomena.
It is actually the RGB channels that are noisy: each channel is noisy mostly independently from the others, and we (humans) perceive that as problems of colors and luminance.
As rvietor said, the distinction allows however when denoising to keep grain without having the disturbing chroma noise.
In denoise profiled, in wavelets, the Y0U0V0 mode use a norm crafted for denoising to separate “luma” and “chroma” (the transformation tries to minimize the noise variance in the Y0 channel even before any denoising).
We could use plenty of other transforms to split some sort of luminance from some sort of chrominance (you can see for instance that we can use plenty of different norms in darktable for color preservation), though most of them were not designed especially for denoising.
As a summary:
- there are plenty of ways to transform RGB to a “luma and chroma” representation, with different definitions, which will give different results
- most of them were not designed for denoising
- the important thing to remember is that what we perceive as color and luminance noise is actually just the combination of the 3 RGB channels that are noisy.