You could be right, I don't know. Your theory is a good one, don't get me wrong. I was just stating what the game developers have said.
Then again, it is ultimately up to Lucas Licensing to decide what really
happened, so maybe we'll be told differently down the road. Until then, I accept what the developers have told us as canon, which has been discussed with and approved by George Lucas: that Vader is loyal at this point.
It'll be nice if a retcon proves your theory correct in the future, though.