[Aubrey's sudden vigor is a little surprising, but Armin can't help but feel a little encouraged himself, seeing what good his words seem to have done. He might not be able to undo the damage that Eren's done back home, but...perhaps he can, in some small way, do so here - for Aubrey, at least.

[He's glad that she's eager not to believe Eren...]

There's no need to thank me. I've only told you the truth.

[After a moment, he smiles, inclines his head.]

I should be thanking you - all that insight. It's a comfort, somehow - even given the circumstances.