No... I'm certain she was sincere.

[He can't blame Childe for his outright disbelief, considering her overall attitudes, but that doesn't change his own confidence in his assessment.]

This world and its people mean very little to her. But from what I've seen, she values her honour more highly than her own life. Her promise may not be the result of any natural impulse towards kindness, but she won't go back on her word.