It is very obvious that Strange never really expected to ever be apologized to and certainly never expected Childermass or Lambert to vocalize anything along the lines of 'hey, we messed up that whole secret dating thing a little bit.' The more Childermass explains, the more Strange's expression shifts to relieved.

So he wasn't mad or out of line to think that those two handled it poorly. Thank goodness.

"Thank you," Strange says again, despite the fact that he literally just said it a few seconds ago. And then to keep things from getting too sappy and too emotional, Strange changes the mood with a little joke: his expression shifts to something pretty dang cheeky. "Though I still think you worried too much about this. I've torn down the mirrors and brought magic back to England—the government and new magicians will have many more things to worry about than who's sleeping with whom."