This is my take as well. I think we can all agree it doesn’t work very well. But it seems intentional.I'm pretty sure it ducks the audio from the driver's side front speaker and sources Siri voice from there. The intention I believe is to let the driver hear the voice without the music dipping, like you mentioned.
This is not the first vehicle I have seen that does this, although perhaps others have options to customize the behavior.
All we need is a toggle button to either do it this way or mute during Siri. EZPZ