I think he sets up a knob on the matriarch to send MIDI to his DAW where he has the delay. Seems like he has the knob controlling the wet dry mix on the delay. Not sure what the delay is he may mention it briefly in the beginning when he gets it going.
I saw him using Valhalla Delay in another video, so i´m pretty sure he´s using this there as well. It has a ducking mode, where the delayed signal gets ducked whenever the original sound is played, this way the original sound never get´s muddied up by the delays.