I'm going to play devil's advocate here, and also echo some points made by Alfheim and Grimm22...
1) As Alfheim rightly points out, Cap has(had?) a history of getting some great shots in on Class 100 characters...he is, after all, supposed to be one of the most skilled martial artists in the Marvel Universe, so one would expect him to have high levels of competency with pressure points, leverage, etc.
While I disagree with the idea of him KO'ing the Hulk, I can see how a writer might logically argue the possibility of such a shot from Cap.
2) As Grimm22 pointed out, Spider-Man in this instance is a very unreliable narrator -- at the time of his remembering the incident, Spidey is grief-stricken with the loss of Cap and the shooting of Aunt May. He's beside himself with guilt, grief and anger, and is probably idealizing the incident in his mind. The flashback we get is thus filtered through Spider-Man's own mental filters, and is not necessarily an accurate representation of any past event.
/ducks behind a wall...