She didn't, actually.
Her argument was that there was no legal requirement to present a birth certificate, so the argument was moot, and the facsimile was irrelevant.
If the judge had ruled against her and required documentation, she certainly would have used the image as evidence. In any case, she wouldn't and didn't "admit forgery."
FTA:Obama’s document forgery and fraudulent presidency have now forced him to flee to a “strange twilight zone” between political popularity and legal legitimacy where poorly counterfeited records are apparently allowed to be published by Obama using government media resources for political purposes, yet those same records are held by the courts as irrelevant for determining Obama’s legal eligibility status because they are, according to judges, “so poorly forged” they are obviously meant to be satirical and not to be taken seriously as evidence.