Amon did do something to Gao Sou. What he used was the Fragile Bone Muscle Poison in a small dose. The poison could make someone lose strength and unable to do anything. In a small dose, it took longer for the effects to show, and the victim would only feel fatigued instead of losing all strength.
Amon didn't lace the poison with the Sky Delight Wine. That would be too obvious. The poison was instead applied to his cup, the one he used to drink. He, of course, had drunk an antidote beforehand, so he was immune.
Amon intentionally came earlier to show off the Sky Delight Wine. Although the venue was changed, the easy-to-recognize wine still drew attention. Once Gao Dou mentioned the wine, Amon looked for an excuse to throw the cup at him.
