How was he playing when he threw meteors on them or when he was using the flower three world and he was about to roast them? How was he playing when he clearly stated he is going to bring down the Hokage first, then he stabbed her and he was sure she died? How was he trolling when he had to send a small army of giant Susanoo to beat them? How was he playing when at the end of the battle he admitted he must use the PS once again to beat them?
He wasn't 100% serious, I give you that, but I'm pretty sure he wasn't playing. Meh, but I guess saying he was playing is easier than admitting the Gokage isn't a bunch of fodders and they actually put up decent battle against him.