AFK player gets S grade

My teammate Jinx was AFK for a half of the game (DC time about the 4th minute and he came back about 20'th or so) We've won so as far as you can see from the screenshots, he had dealt no damage, he has one kill and several assists, but he has got an S grade. I'm not jelaous, just questioning, how the player w/o dmg and with a very poor KDA could have such a grade? [scr1]( [scr2](
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