Each individual is the only being in the universe who actually know what they perceive, so how could it be a lie?
Because he didn't see an intruder and so how could he say he was pretty sure he was armed? He has said that he feared for his and Reeva's safety and that there might be an armed intruder in the house but to say he was pretty sure there was an armed intruder would surely be a lie. Where does his lack of glasses help? If his eye sight meant he couldn't see clearly how could he say he was pretty sure of an armed intruder? To have taken the line you suggest would have been ripped to shreds by the prosecution I would suggest.