It's essentially both but really being driven by Apple's dominant position in the mobile OS market.
I find it similar to the Microsoft DOJ case in the 1990s which wasn't so much about browsers but rather the desktop OS market which MS dominated and their (alleged) anticompetitive behavior related to Windows access, restrictions, etc. of things like alternative browsers.
In the Apple situation here, it's about the mobile OS market in which Apple has a dominant position and their alleged anticompetitive behavior related to iOS access, restrictions, etc. of alternative app stores.
If Microsoft only had 20% share of the desktop OS market, there wouldn't have been a case. If Apple only had 20% share of the mobile OS market, there wouldn't be a case here.