I don't understand if it's just to give their new ARM chips a performance edge over x86 based cpus
That wouldn't make much sense, since they are the ones selling the x86 MacPros. They just wouldn't have added RDNA 2 support in the first place if they wanted to have the ARM Macs being the top dog.
Additionally, currently they anyway sell every M1 Mac they can produce, no need to do something that would hurt their reputation for nothing.