Absolute garbage. Once "YouTube Audio Mode" is activated, the video continues to render at the set resolution of 1080p or 4K60. You can check the GPU usage in the Task Manager; it's 10-15% depending on the video resolution. But if you disable the magical "YouTube Audio Mode" and manually lower the resolution to 144p, GPU usage drops to 0.4%.

It simply displays a static image, but the entire video continues to consume resources in the background.

Fake. The bandwidth is the same as watching 144p video but without video. Doesn't make sense.