The way I understand it, he's using a clamp-on cable hanger against the top bearing, being preloaded by the top nut from the old threaded headset. Once the preload is right, then he tightens up the clamp to 'lock' it.
This is on a threaded steerer, using a quill stem. Once assembled, it looks and functions just like a threaded headset.

Again, unless you've got some weird size that is out of production for decades (1-1/4" Threaded MTB?) I'm not sure why this would be a preferred setup over the correct threaded headset. 1" is still widely available, in from Sunrace to Campy Record, and 1-1/8 threaded are still out there, if not as common.