I agree that it does undermine her credibility, but I think it only undermines it seriously if you take the story out of context.
Mrs Diallo lied about being gang raped in order to escape a dangerous country where gang rape is an everyday real risk to women. She didn't lie about this in order to create a false rape charge. She also, poor thing, couldn't have known that she would be the victim of rape in the future and have this action used against her in order to have charges dropped.
More context - forensic evidence proves beyond reasonable doubt that a sexual encounter occurred between DSK and Mrs Diallo. She says it was rape, he says it was consensual.
They have both lied about things - she lied in order to flee a country where she was under risk. He lied about whether he had a sexual encounter with her in the first place.
Seems to me that his lie is a whole lot more relevant to this actual case than hers.
Her lie about the gang rape had a perfectly understandable motive.
What would be her motive for lying about whether DSK raped her or not?
What would be his motive for lying about whether he raped her or not (or indeed whether he had any sexual encounter with her or not)?
Mrs Diallo may be considered to be too weak a witness. However, I think she is only a weak witness because the current legal system makes rape cases very very antagonistic where the point isn't to find out what really happened but to discredit the woman and get the man off at all costs. Rape victims are still being put on trial themselves even though it is officially illegal to do so.
I blame the patriarchy.