Color spaces

Hi everyone,

Does anyone here know enough about colours spaces to understand the implications of this issue?
https://github.com/w3c/wcag21/issues/815#issuecomment-378463427

Copying it here:
...the Understanding WCAG document should clarify is whether "relative luminance" should be considered equivalent to the Y component of CIE XYZ (especially for non-sRGB color spaces such as DCI-P3) (more specifically, equivalent to the "luminance factor" as defined by the CIE) and whether the notes in that definition specifically apply only to companded ("non-linear") "sRGB" colors encoded as 8-bit-per-component colors and not to sRGB or sRGB-like colors encoded or represented in some other way (whether by using different red, green, blue, or white points, a different color component transfer function, a different encoding form, or otherwise).

I understand there are different colour spaces that display colours in varying ways... but this question has lost me.

What impact would that clarification have?

-Alastair