You'd think mentioning 'again', specifically, would put Popp more on guard and not less. However, he knows a lot about wanting to make up for your mistakes, from every single perspective he can imagine, and now he feels really bad for snapping at Rosie, along with the strata of still being mad about being poisoned and a little scared and also now he feels bad for feeling that way, and yelling at her didn't even make him feel better, he just feels like an asshole now!

"...I mean, I am pretty stupid, I guess..."

He steps closer, shaking from nerves. His eyes are wet around the corners, even if he's not outright crying. She might try poisoning him again, honestly, and then he'll feel really stupid. He'd rather feel stupid for trusting people than for not trusting people, is what this comes down to.