She’s totally right. I always understood that look Cas makes as “sincere” since he’s trying to explain to Dean he was saved by The Good Guys and that everything’s going to be alright. But no, looking at this scene again, the camera really lingers on Cas’ face, which begins to look more worried, than surprised or sincere.
In those first few seconds of their meeting, Cas really does act like they should know each other already and he has no issues with saying his name, because, of course, all his brothers and sisters can’t remember his name sometimes and Castiel’s wearing a vessel now, so Dean wouldn’t be able to reconigze him anyway. Then Dean asks, “But what are you?” And BOOM Cas stops friddling through the book, he looks over, pauses, and explains it.
Dean must have asked him these same questions in Hell. Rewatching this scene, I can clearly see it now, that Cas is thinking, “Okay, I guess we’ll have to start over, he doesn’t remember me.”
Kind of reminds me of The Born-Again Indentity where their roles were reversed and Cas was the one who no longer remembered Dean, forcing Dean to act like they’d just met too.