I think they are talking about the Steele Dossier, not sure though. 

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Donald Trump Loses London Case Against Ex-MI6 Spy Over Kremlin Dossier  https://www.bloomberg.com/news/articles/2024-02-01/donald-trump-loses-london-case-against-ex-spy-over-kremlin-files 

Donald Trump lost his London lawsuit against an ex-British spy after a court dismissed his claim linked to the notorious dossier about alleged ties between the Kremlin and the former US president’s successful run to the White House.