"Childermass," Strange simply responds, still unconsciously rubbing his shoulder. And that explains how he got caught off guard. Of course Strange wouldn't expect Childermass of all people to shoot him. Still, he feels the need to defend the man, to explain the circumstances, even if they both already know that Childermass in his right mind wouldn't even think of shooting Strange.

"He was one of the people who were kidnapped by the Agents—and don't ask me what they did to him, for I haven't the slightest clue myself." Of course Childermass wasn't going to talk to Strange about what happened in the Matrix. To start with, he was Childermass, wringing water from a stone was easier than wringing a conversation about himself. Even now Strange doesn't have any idea about what happened or the full extent of what the Agents did to him in the Matrix...just that it must have been terrible.