Comment by pwg

It was, but xdamage is part of the composting side of the final bitmap image generation, before that final bitmap is clocked out to the display.

The frame buffer, at least the portion of the GPU responsible for reading the frame buffer and shipping the contents out over the port to the display, the communications cable to the display screen itself, and the display screen were still reading, transmitting, and refreshing every pixel of the display at 60hz (or more).

This LG display tech. claims to be able to turn that last portion's speed down to a 1Hz rate from whatever it usually is running at.