The Washington Post last year confirmed that two intermediaries had relayed such a claim to lawmakers and the FBI. The Post did not publish a story in part because the intermediaries declined to identify the alleged witness and because the woman who was said to be involved declined to comment. The Times article, drawing from reporting for a forthcoming book, is based on interviews with “two officials who have communicated with Mr. Stier.”
https://beta.washingtonpost.com/national/democratic-candidates-demand-kavanaugh-impeachment-after-new-allegation-in-times-piece/2019/09/15/3bdc160e-d7ce-11e9-ac63-3016711543fe_story.html