This seems awfully nitpicky. The Wiegman film shows a puff of smoke hanging over some of the trees on the knoll. The fact that Holland wasn't certain which shot corresponded to the puff of smoke does not change the fact that he said he saw a puff of smoke on the knoll, especially given that other witnesses also said they saw a puff of smoke. The smoke could not have come from the steam pipes because they were too far away, nor could it have come from the moving patrol bikes.
Plus, a number of witnesses in the motorcade and on the knoll said they smelled the pungent odor of gunpowder near/on the grassy knoll.