Not at all. I just read the evidence as presented to the court and think the balance of probablities lies in favour of Pistorius' own description of his motives being correct. I saw nothing that ruled it out, and as none of us can mind-read, I accept (as I think the judge did), that he thought he was confronting an intruder. That doesn't for a second change my view that he committed murder.
I think there are several very solid reasons to suspect that Pistorius must have known it was Steenkamp behind that door. I think that his whole story simply doesn't stack up in relation to normal human experiences. He would have had to have walked right round past the bed on his way to investigate the noise, so he could (and, IMO, would) have checked that Steenkamp was in bed, and whispered or said to her to stay there and to call the security staff, before heading off to the bathroom armed with a high-power handgun.
In addition, by definition (if Pistorius' account is to be believed) there HAS to have been sufficient opportunity for Steenkamp to have slipped silently out of bed and walked to the bathroom area without Pistorius seeing or hearing anything - even though he says he was only bringing the fan in over a matter of seconds.
And also, I find it very hard to believe that Pistorius was (per his version) standing in the bathroom yelling "Get the ... out of my house!" directly towards the closed toilet door, and yet Steenkamp wouldn't have made some sort of calling out to tell Pistorius that it was her behind the bathroom door (given that if Pistorius' account is to be believed, Steenkamp would likely be aware that Pistorius hadn't seen or heard her slipping away from the bedroom to go to the toilet).
And then we add in other evidence of Pistorius' personality and his treatment of girlfriends, and I conclude that by some distance the most plausible narrative is one where Pistorius and Steenkamp had an escalating argument in the bedroom area, Steenkamp started to dress to leave (perhaps in response to Pistorius yelling "Get the ... out of my house!" at HER), Pistorius physically threatened Steenkamp in the bedroom area, Steenkamp dropped her jeans (as they were found in the bedroom) and ran to the toilet, locking herself in, Pistorius followed her on his stumps, then Steenkamp said (through the door) that she had her phone and she was going to call family/friend/security/police, tell them the threatening situation, and request assistance. I think this would have pushed Pistorius over the edge in rage - the combination of Steenkamp's unequivocal decision to leave him and (maybe more powerful) the thought of having his image and reputation damaged by a public accusation of abuse and physical violence towards Steenkamp. I think he really did fire somewhat reflexively at the point, in a blind rage - and almost immediately realised what he'd done and was overcome with remorse and grief.
But as you point out (and as I have done too), the issue of Pistorius' presumed identification of the person behind the door is wholly moot in the context of the criminal charge and the conviction. And as you also say, it's impossible to prove one way or the other whether Pistorius genuinely thought he was firing at an intruder or Steenkamp. That will be cold comfort to Steenkamp's family though, I suppose. I am guessing that all of Steenkamp's family believe that Pistorius knew it was her behind the door (not that that makes any difference intellectually or legally of course).