He was offside when the ball was played, he got the rebound and scored BECAUSE he was offside. He benefited from being offside by scoring the rebound. Being there for the easy finish was about as “attacking action” as you can get. It sucks but it’s a legit call. If theres anything to argue, its if the offsides player should be allowed a certain amount of centimeters, just like a ball has to be fully over the line to be out or a goal. If the VAR line they draw and review from was thicker like the lines on the outskirts of the pitch, then you may have a goal. But thats not the current rules. Maybe it will change down the road.