Yeah that would do it.
*goes to quickly rewatch that little bit* Yeeeah there’s a certain amount of difference between “Woo Cas is here!” from Dean before that and then “Oh good you’re alive that’s something anyway pal” after that comment, because he was smiling wonderfully at him when he came in and even as he walks past him with the pizzas but then…
Yeah. That muted response everyone’s been so upset about comes after Cas brings up that Hannah did the thing and saved him.
For the sake of argument Dean doesn’t know Cas is lying to him at all so his subdued reaction has to be something else, because if Cas’s crappy lying blew the whole thing open, Sam’s arc would be over before it started. There are major plot reasons why Dean can only be disappointed by this not because he feels he is being lied to but because the statement itself upsets him. :P
Urgh this is awful. I didn’t stop to muse on this at all until now. (Thanks. >.<) This is a DELIGHTFULLY soap opera plot:
Hannah expressly wanted to do exactly that in 10x02 because of her awkward new ~feelings~ for Cas, Cas was like please don’t, so the lie Cas uses is romantically coded to the viewer, and Dean’s previous suspicions about Cas and Hannah are at least on the table thanks to the end of 10x03.
And then we got the contrast in the actual plot, where Cas liberates Metatron to try and save Dean twice, once with Hannah’s permission and once without. Fracturing his relationship with Hannah in the process by doing exactly what he said he wouldn’t do for himself, or because of her, or let her do because she loved him, where it all changes when it’s for Dean. So his priorities are clear to him (and to Hannah - though she seemed to always have a fear about said priorities) and would have been to Dean if he could tell the truth.
Anyway then when that is all over he’s left with a choice about what to do with Metatron and decides (probably because he’s already so deep in trouble for stealing him in the first place that he can’t exactly get in worse trouble for borrowing him a bit longer) to do the empowering thing for himself (well at the end of this episode he said it was selfish but it was treat yo self day I’m not letting Cas think that :P) and use him to get his grace back (because that quote about not letting himself die) and he loses Metatron in the process.
So exactly what he didn’t want to risk happening when Hannah wanted to do it happens (and there’s speculation that Metatron wanted him to find the grace because of probably awful reasons) when he tries to do anything for himself / for Dean (getting his grace back does also benefit him helping Dean in the long run), so he’s running at negligible victories anyway…
He heads on home and Sam has to remind him with “you know what to say” eyebrows when Cas turns to him to see if they’re still lying or not, that Dean doesn’t know and now can’t know about anything they did because revealing that they did something behind Dean’s back once will make him suspicious about Sam going behind his back again and Sam’s very busy now going behind Dean’s back to work with Rowena and Cas is trapped in the crossfire of maintaining one lie so Sam can carry on the other…
Not that Cas knows the last part: he thinks Sam burned the book as well. But in that conversation before Charlie and Dean come in Sam seems to be bonding with Cas without his knowledge about them making poor decisions like “Oh well he let Metatron run free, anything I do after this is at least two makes company.”
Cas gets the solidarity and sympathy for his actions without really knowing why so it puts Sam emotionally on his side even more, inspiring Cas to continue the deception, and so to keep himself from even risking Sam at all in this he puts Hannah into the story in Sam’s place… Hannah who Dean was suspicious of and then tried to kill him but then Hannah who Cas spent months working with and though they’ve been low on communication all season (probably because of the ongoing miscommunication) Dean would at least probably know what Cas was doing in that gap of time where he was with Hannah.
And so we’re at a place where Dean thinks Hannah did what she wanted to do in 10x02 (giving this a weird narrative callback to remind us that it was because Hannah had ~feelings~ for Cas and why this is then a terrible thing to say which at least gives us extremely plausible motive for jealous!Dean because of him having feelings too) and from Dean’s sudden dull response and the callback to Hannah wanting to be the one to save Cas, suddenly we have it as “Hannah did this to save Cas because they have ~feelings~ for each other” since Dean doesn’t know it isn’t like that…
And so now Dean thinks Cas has been in a long-term relationship with Hannah possibly since season 9, slight hiccup in 9x22 aside, for all he knows.
But on the plus side:
Jealous!Dean does give us a nice dose of actual Destiel feelings from Dean’s side even if he’s being more than usually green eyed.
The lie taking on romantic coding towards Cas and Hannah goes in direct opposition to the actual story, and so we have the fan fic trope of mistaken “he’s straight and taken” possibly playing out in front of our eyes.
And if Hannah 2.0 is seen again or described to Dean then we get Dean realising that’s who Cas has been in a long-term romantic relationship with. Ehehe.