To summarize:

When a prosthetic lower leg was attached, they connected antagonist muscles to the leg, with sensors from those muscles in a control loop to the leg (ankle), mimicking how proprioception works. (The sensors are the new interface technology.)

The patient knew and could move the position of the foot when he couldn't see it. He walked up stairs with the usual natural coordinated movements. And he felt like the leg was part of him.

It's one thing to (cortically) plan and execute and track prosthetics visually; it's another for the cerebellum to autonomously monitor and control them, and not to feel cut off.

This seems workable as a standard of care for arms and hands as well. And in this case, it was installed years after the leg was lost, so it works for retrofits (granting this is N=1 young patient in otherwise excellent condition).

Osseointegration was another example of interesting real-life cyborg technology that I stumbled upon.