Istredd's quiet for a moment but he's not upset by what Lucifer said. Stregobor was a bad person, he's the one who told Lucifer that, backed up by many examples of his villainous behavior. So his instincts and reactions to this are reasonable.

I wish I only remembered the bad things about him. I wish my feelings about him weren't so complicated.

It would mean that this never bothered him in the first place. It wouldn't be hanging over his head. Istredd could have shrugged off his death and gotten nothing from it other than 'good.' But that's not the situation he's in. He knows Lucifer understands given his own situation, and the last thing he wants to do is point to it, poke at his parental trauma just because Istredd's in a bad place.