If she asked why, he might even tell her, but he wasn't sure he could volunteer the information. Severus had gotten by the last 17 years by avoiding confronting his mistakes more than was necessary in order to keep going. If Lily didn't know, that meant Dumbledore really had respected his wish to keep it from her, and if Harry hadn't told her either, then he also respected that wish. Which was more than Severus had ever given him.
"It was," he insisted, "I knew about it, I--" He paused and looked down at his hands, the next words coming out more slowly, softer, but she deserved to have as much of the truth as she was willing to hear. "I was the reason he knew about the prophecy in the first place. I heard it and I told him. I didn't know he'd think it meant you."