The iGPU's video decoder could even be drastically less efficient than the dGPU's decoder, and it would still be better to use the iGPU, because just powering up the PCIe link and VRAM for the dGPU costs far more power than the video decoding itself.

Exactly, mpv logic causes it to pick up Nvidia for HW video even if renderer is on iGPU. There is special flag to choose device which solves the problem and Nvidia is kept down