That’s interesting. I think this is one of the threads that I vaguely recalled in which people discussed issues with iOS volume controls (issues with iOS device volume and Roon volume jumping around). Nothing extension involvement:
You don’t need to educate me on how extensions work 
There are obviously gaps in my understanding. Maybe there’s a formal “volume extension” protocol in which extensions such as Deep Harmony can take ownership of volume. That would make sense. I’m pretty sure that isn’t what extensions such as Home Assistant and rooExtend to, though - they seem to just be making volume adjustment API calls.
I jumped to the conclusion that this was user error on @Anthro’s part and I was wrong about that. Sorry for the misinformation.