Lol, I’m sorry? But how else is he supposed to react when she said that? Tbh, I don’t find his reaction very weird at all.
We understand her and recognized her words because we watched Aideen experience life and death and rebirth, but imagine yourself in Dion’s shoes. In all honesty, if the MC was Dion, and this girl said something like this, we’d probably think it weird as well. I’d get the implication since I’m a reader that reads a lot of rebirth/transmigration stuff, but I’d think for a moment that she was absolutely bonkers LOL
IMO, I think Dion just didn’t know how to respond verbally, so he patted her arm and said she was admirable because he knows that she probably experienced something beyond his understanding. I bet he’d definitely ruminate on it or wait until she tells him more.