Qualcomm has been using Arm cores for several gens now instead of their own. Nuvias core will the first custom one for 'em in quite a whileApple licenses the ARM ISA and possibly some IP, they and Qualcomm have their own microarchitecture teams, the core designs are pretty much useless to them. Samsung ditched their team, but their cooperation with AMD is a bit of a sunk cost so they aren't going to be entirely happy with NVIDIA becoming a defacto mobile standard.
Greater competetiveness of competing microarchitecture creates margin pressure for Apple by more competition on the mobile market. Their hold on customers is very strong but not absolute and there's always this small chance that an openly available competetive microarchitecture would let a third competitor on the integrated consumer electronics ecosystem market to arise. Better to cut it off at the knees by crippling a competing microarchitecture.