Exactly: the same signal. If the electronics controlling it receive one input from the steering wheel and a different input from the self-driving computer, are you sure it will prioritize the steering wheel input in every single possible circumstance? 'Cause I’m not!
That's an interesting point there, but have you considered that even with a mechanical link and current safety features, it can still override you? I unfortunately almost drove into someone at very low speeds in a dark rainy parking lot, but the cars safety systems overrode me thankfully. I don't think they would have been injured it was so slow, but just to show that nowadays with cars you don't always have full control. In that case it was the brakes not steering, but modern cars can and will prevent you from changing lanes into someone in your blind spot for example.
All of it, mechanical link or not, ultimately comes down to software now, and I'm not sure there would be any real difference between your steering wheel input overriding a self driving system with or without a mechanical link. Putting torque on the wheel while in these semi self driving modes disables the self driving features, but that's software that disables it when you take over. What if that software failed and you were now fighting the self driving car also trying to steer and as you tried to steer it put equal power against you thinking the steering was rough?
Also with steer by wire if there is a catastrophic failure, turning the wheel will simply do nothing. E.g if both motors simultaneously die, or somehow all 3 signal wires at the steering wheel are cut. Edit: I believe it's dual battery redundant as well, so the HV and 48v would both need to die to kill steering.