Daryl’s lie - and Carol believing it
In all honestly, I believe that, because of the way he hesitated to answer, and because he then said twice that everyone was okay, as if he wanted to make himself believe it as well for as long as he’s there, Carol absolutely knew that Daryl wasn’t telling her everything, that he was holding back on something. Carol has known Daryl for two years now. She knows him well enough to gauge his reaction to her question, and to know that his answer was not truthful.
But she deliberately chose to take his answer at face value at the time, simply because she knew why he was holding back or lying - to protect her, and give her the time to heal that she had told him she needed (”I had to”), and come to terms with what you have to do in this new world to survive.
So she chooses to accept his answer without further questions because she realizes what it means to a man like Daryl, who says what he thinks, who has never lied to her until that moment, to lie about this, and she doesn’t want to throw this gift away that he is giving her - this gift of just a little more time in her bubble, a little more time away from it all to gather herself, to heal as far as possible, to arrive at the conclusion that, by staying away voluntarily, she has already lost the people she loves, and that she might keep them alive longer by joining the fight, and that keeping people she loves alive by killing those who have no other goal in life but exploiting, hurting, and killing them, does not make her a bad person.
It’s a gift of time that Daryl is giving her, and time, immaterial as it is, is the most precious thing we can give the people we love, and she appreciates this gift and accepts it from him, knowing how huge it is.
They are so perfect for each other.