He dies after one hit makes contact. He just dies off screen. This is supplemented bot by the fact that Sans makes the 'death sound' and he asks Papyrus what he wants from Grillby's, who - in order to be fighting Sans - you had to have already killed.
could be true but you also gotta think about it this way, a fake attack would do about 1 damage, now I wouldn't put it past undertale to make a packet of ketchup to have 1 hp, would you?