I think it would be easier to argue that Apple is being anticompetitive by having undocumented API's only their apps can use, and by not allowing end users to change default mail, map, and browser apps etc.

That would be horizontal integration. But that’s even a hard argument to make since Apple doesn’t have a monopoly or large market share of any of those segments — remember how Apple’s growth was supposed to slow down since their customer base has been saturated? Maybe luxury products but that’s a stretch. Apple isn’t vertically integrated since they outsource some of their manufacturing and assembly to other companies.