I'm not sure why you think Pistorius had to understand anything about legal definitions of intention or common everyday use of the word intention. All he needed to do was say that he shot on purpose, that he meant to pull the trigger, that he wanted to shoot to protect himself - he failed to do this every time he was asked in many different forms, in fact he didn't just fail to say he wanted to fire the gun, he said he didn't want to, he denied wanting to, he said that it just happened without any thought or self-control or decision or will, that it was accidental or automatic. You can't get further away from intentional shooting than that - it is the opposite. The problem for him is that the psychiatric evaluation decided that he was in full possession of his senses and his actions.
It is faulty logic to think that because Masipa found he did intend to shoot, it satisfies the requirement of intention to shoot by someone who is claiming they were acting to save their own life.
For PPD to succeed, one of the six requirements is "the force used was directed at the attacker".
Pistorius said he did not aim at the (perceived) attacker.
If you can see why directing force at the attacker is an essential component of PPD, (you can't be acting to save yourself if you didn't try to stop the attacker) you must also be able to see that Masipa's finding that he intended to shoot does not assist him. In other words she said he lied when he testified he didn't exert any will or control over pulling the trigger 4 times. A finding of untruthfulness is by its very nature an adverse finding, it cannot be used as evidence of a lawful defence to a criminal action an accused person never advanced themselves.
As the State says "The Applicant's reliance on putative self-defence does not get out of the starting-blocks, since on his own version he never intended to shoot at the perceived danger", and the State again referring to what the SCA said, "The Applicant... has totally ignored the insurmountable hurdle that he on his own version ..had not intended to shoot the person whom he felt was the intruder... which immediately placed him beyond the ambit of the defence.."