Maybe it’s using Wake-on-LAN where the network interface is alive while the rest of the system is dormant? Perhaps the music stream is encoded in magic packets?

I found [1], which says "For Qualcomm hardware where the USB audio can be offloaded to a dedicated audio DSP for handling the transfers to the USB host controller, this lessens the work of the main CPU cores and can help with power management so those CPU cores can hit lower power states or tackle other work. Other hardware vendors will hopefully follow suit in their support for the upstream support around USB audio offloading."

So I think that it doesn't do anything for the average Linux laptop, and is mostly intended for certain (mobile) hardware?

[1]: https://www.phoronix.com/news/Linux-6.16-USB-Audio-Offload