Means this is what Sasuke wanted Naruto to do from the beginning i.e. use his full power, which Naruto didn't do. But at that point when Naruto went full power due to Sasuke going full power, he did what Sasuke wanted which is why Sasuke was excited about that. I already said if he did all he could, then it doesn't matter if he was serious or jolly. He countered every attack relative to Sasuke's attack. And when Sasuke went full power, he also went full power. Proves that Naruto couldn't overpower Sasuke if he wanted.
And yeah, he wasn't trying to overpower Sasuke because he didn't want to kill Sasuke. Not that he could kill him BTW. If Naruto tried to overpower Sasuke, Sasuke would've responded with an attack of similar power, so no problem there.