While your guess is as good as mine. I can see that the screen sharing feature goes beyond what is currently possible with 3rd party mirroring tools, including apple's own earlier tools. For example right clicking brings up extensive contextual menus that aren't accessible in iOS, and I can see these also leverage the continuity features between the platforms.
I can install a whole number of AppStore or opensource apps that allow me to access other machines graphically. I really don't see why accessing the GUI on an iPhone should be treated any differently than accessing the GUI on a terminal server or an android or linux box or something.. The argument doesn't really make sense to me..
I’m unsure whether the DMA compels them to provide specific APIs beyond the ability to connect to arbitrary devices and draw to the screen, and it’s maybe a little bit concerning if it does. My understanding was that nothing in the DMA specifically compelled Apple to create e.g. MarketplaceKit, it’s just that the alternative would be to open up iOS far more than Apple is willing to do.