No amount of distance is a boundary. People can travel, use cell phones, or any other method to get past distance. It is only a logistical complication.
Psychological and emotional boundaries are required. Cutting off contact by itself is not enough. You have to communicate that you are cutting off contact. That's what establishes the boundary. In many cases you won't get a restraining order if you haven't established that boundary. The restraining order is a tool for physically enforcing an established psychological and emotional boundary.
I agree with you, seeing how even my spouse's 16h-drive-away mother is still managing to insert herself into her child's life several times a day. I can see on my wife's face she's been talking to her mother when I get home. Even with new recent boundaries set (we're not visiting anymore, no more answering the phone or WhatsApp 33 times a day and then when not answering right away having other family call to argue that you're abandoning your family, fuck all of them), and I feel my spouse has made tremendous strides there, the mum is still finding ways to do this.
Distance and cutting off contact (restraining order if necessary) are profoundly effective boundaries.