Watching the video won't confirm anything unless you positively know what the pilot is doing with their left hand. More likely whether or not it is coming in or out of power is that due to the split throttle nature of all twin engined fighters I can think of, that the pilot either decided to just use a little bit from the left to get up to speed of those he's following, or if they're coming out of power, they haven't come to idle perfectly synchronized. No crazy engine logic or programming, just manipulation of left and right engines with very fine movements from the left hand.However, the plane just completed a left turn, and right engine was supposed to give more thrust to create a torque. Without the use of afterburners, the left engine would generate more thrust than the right one that had a fully opened nozzle.
My guess is that before the turning, the pilot applied brakes. During the turning, he increased the thrust to catch up the speed and both engines' nozzles closed in. After the turning, the thrust needs to be reduced back to almost minimum. it seems that J-20 is programmed to open up the right nozzle first then the left (to avoid sudden change in the thrust, which is more important in the air for control stability). Of course, we need to watch the video to confirm the theory.