Did he truly drink punch? Or what he thought was punch? Yes, he truly drank punch.
Did he drink the punch before all the ingredients were added/spiked? No he did not drink it before all the ingredients were added.
Was he the one who spiked the punch bowl? No, he did not spike the punch bowl.
Why did he leave early? Was he the murderer? No, he was not the murderer. First question was not phrased as a yes/no!!!
Did he pass on some deadly virus/bacteria by drinking the punch? Nope.
Was he responsible for causing their deaths somehow? No, he was not.
Did something happen at the party (ie plane crashed into the house) after he had left? No, no plane crashed into the house.
Does the punch have any relevance to the riddle? Yes, the punch is relevant (wise for asking!)