Good point on DX12, they only work on DX9 and DX11.
The findings in the video make sense to me. If your GPU is idling then it's more likely that it can immediately start processing a frame submitted by the CPU. If it's running all out then there's a greater chance that it's still busy with the previous frame and therefore the CPU needs to wait a bit.
However, this only helps with latency not overall visual clarity and smoothness of the presentation which are also important. One strange thing about the video is that frame capping reduces latency by multiple frames which doesn't really make sense if ULL mode is working as expected. At best it should reduce latency by less than one full frame vs ULL mode.