You're suggesting that there is no other appropriate handle they could have come up with beyond "sussexroyal"? And beyond that, what gives one person the right

You're suggesting that there is no other appropriate handle they could have come up with beyond "sussexroyal"? And beyond that, what gives one person the right more than any other to a particular handle? Even with your POTUS example. So yes, I think they should have at least asked first and tried to resolve it amicably.