I contacted my old A-10 driver friend who replies:
Nice vid. That looks like where I went through A-10 RTU. It was a gruelling course where they crammed 2 weeks of training into 4 months.
Start the video again and be ready to pause it quickly at the 8 second point. You'll notice he has 2.75 inch rocket pods outboard of his Maverick missiles.
What he did was arm those stations and the gun. When he rolled in, he armed his master arm switch which made the gun and both rocket pods hot. The sight for the rocket is different on the HUD display from the gun sight. He lined up the rocket sight and pressed the pickle button on the stick and one rocket fired from each pod. He then transitioned to his gun sight and pulled the trigger on the stick which turned on the gun. The rockets launched first but the cannon shells were so much faster, they almost beat the rockets to the target.
It's an odd weapons combination for that target set. He might have been marking the target with the two rockets for other aircraft while at the same time suppressing the target area with the gun.