None of that is relevant. He wasn't trying to run - nobody thinks that he was. It is also irrelevant that he thought the ball wasn't in play as that is also made clear in the rules - it is in play until both teams consider than it isn't. Where he thought the ball was or wasn't doesn't matter - the fact is, when he scratched the ground the ball was in motion towards the stumps. It was in play as Carey didn't hold it. Jonny then stepped out of the crease and the ball hit the stumps. The ball was in play the whole time but Bairstow himself didn't know that because he wasn't watching it. And that is why he was out.Where he thought the ball was is only relevant to this discussion is if you're saying he thought the ball was in play.
For arguments sake, he guessed and or assumed. Where did you think he guessed or assumed it was?
Surely not anywhere but in the keepers gloves or he would have taken a run?