Basically she's telling her five year old firstborn that he wasn't important to her. I'm sure he really appreciates her honesty.
I think it's a hell of a lot better to say "Yes I fucked up and could have done better by you" instead of making up something because it's what people want to hear.
His Father will have told him exactly what happened and she can't change it.
If I were her son I would want to know the truth and make my own mind up about it (which he clearly has), rather than have a mother who left me then lied about it. Insult to injury IMO.