It would prove him a liar once again, and add to all we have that speaks against him.
You seem to believe that they've always had access to Michael's private life....I find that hard to believe considering they have no access to their daughter's. In the beginning they believed he loved her and they didn't pursue any information to the contray. Why would they?
But either the Schindlers don't want to use it, or it's not true. Personally I feel it's not true, because it's been 13 years and all we really have is an accusation from someone who is suddenly "not at liberty" to say any more.
Have you considered the possibility that it is true but they don't have proof that a court would consider? Think about this...there are two women who have come forward, both have been harrassed, they're so afraid they sign affidavits but won't go to court...now we have a woman with 1 or 2 children and no one hears a word from her (and you wonder why??)