That’s the point, there is no competition to harm as there are no competing operating systems!
There is, they can harm competition in other verticals. The Spotify case is an example of this.
Spotifys main complaint being that Apple were using their app review process to dictate how they comunicated with their own customers and became increasingly agressive in rejecting their submissions once they had launched a competing product (Apple Music).
The crux of this case in the UK was that Apple and Google were obstructing cloud gaming services. This is problematic as they have (had in the case of Google Stadia) services that could be considered competitive.