The corners DO indicate the flashback, but Doc said it was in how they "GREET" him, and THEN we saw the shock. If he had said it was in how they "GREETED" him, then I would have expected the reaction to be a flashback or memory.
For that matter, I'm surprised we haven't gotten a "who is this Morty?" reaction again. The relatives I knew when growing up call me one thing, and my wife and kid know me by a different name, so I'm actually familiar with that, not even counting screen-names and so on. (Right now, IRL, there's people referring to me by 4 different names, and none of them is the screen-name I'm using here. )