How do you mention you can play multiple audio streams at the same time and then claim the OS is designed to let only one app access an audio channel / device? Which one is it now?

Just because it's designed that way doesn't mean it works that way all the time.

Basically they're saying it's designed so that it's easy for an app to override your current audio stream, but allows for it to run concurrently if they want.

As most app developers wouldn't want that, they hijack the audio by default.