Not nonsense. Lizzy and Mika's father extracted a promise from Carol. She said she would look after them and protect them. Carol "failed" Sophia. She comes across Lizzy, whom she castigated as "weak" who has done this deed and Carol is no fool. She knows how the group will likely react. She failed one daughter. She feels she can't fail again with her new surrogate child, to whose father she made a solemn promise. Her commitment to her new daughter overrides her self imposed mission to safeguard the other kids.
Psychologically, I could see it. TPTB may not go that route, but it's not so implausible. I still found Daryl's reaction most interesting. Him wanting to get Tyrese's reaction immediately to see how they could deal with the situation--and he didn't entirely buy it. He thought something else might have spurred the situation. All I know is, Carol simply can't be gone for good. There's too much story.