I can't imagine. AFAIK no phone has "grayscale" pixels. Grey or white light is achieved by blending the other colors equally.

A true monochrome screen is a thing, but they are for specialized applications and not used in phones.

Maybe power draw is not linear and with grayscale rgb sub-pixels can operate at lower intensity and lower power draw than if the luminance wasn't distributed across all colors?

They said they noticed themselves looking at their phone far less. Most likely this is what saved the power, just a typical spurious correlation and a bad theory for the reason.

some oled displays have white subpixels, i dont know if the power cost is a thing though