I am sure there is a reason, but I wonder why ther was not some sort of shut off valve located within the BOP stack that could be manipilated by a ROV?
I have no idea, but I think from what I have read that they relied totally on the auto shut down systems.
From your name, I would guess that you write code.
Before I retired, I sold equipment, some of it designed by my empoyees, that was operated totally by computers.
On no machine would I allow a design that did not include a mechanical back up.
So your insticts are correct. In some manner, there should have been a mechanical device located upstream of any possible pipe failure.