The assists were probably just a couple pings on it. I've taken the D3A up against planes in Early war and owned spit5's easily.
Not necessarily. I flew the A6M2 a ton last tour to prepare for Rangoon, and I when I could get on someone's six and sustain BB fire on them for a while, I could shoot them down. The problem would be that just one "friendly" zooming by shooting one cannon round into the same plane would get them the kill, even if I did all the real work.
Furthermore, and this is either my imagination or the game is modeled better than we realize... I'd ping planes 600 out with the BBs, see hit sprites, the target would get shot down by someone else, and I wouldn't even get an assist. I'm guessing that after a certain range the .303s have so little momentum left the game models them as bouncing off, literally no damage. Either that or I imagined the hit sprites in the first place. But it happened too many times for that.
So either way, his kills/assists ratio seems correct.