Just thinking about "It's an inside thing" and how important that is to me.
Mel who, as an (potentially undiagnosed?) autistic women, has rarely felt 'in' on the joke, often taking things too literally and not reading tone particularly well. And here is Langdon who not only validates her joke (which is also a call back to joke made 10 months ago) but also frames it as something that connects the two of them. And in not explaining the joke to Dana (not that he has to, she probably wouldn't care) he is letting other people be on the outside for once.
I can just imagine a moment in s3 (or later) where Langdon and Mel are talking and Langdon makes a joke reference to some niche interest or a previous conversation between the two of them and Mel understands what he is saying and continues the joke, and everyone else is like "wtf are they talking about" but Langdon just smiles and says "yeah you get it!" and Mel feels so proud and validated!
