What they don’t want is for a Special Master to review the “classified” documents because they don’t want a decision made that they have been properly declassified and are not a national security threat. As is normal in court cases involving classified documents/information the court can seal the “evidence” from the public and deny the defendant the ability to argue the classification of the document. If a Special Master rules that the documents are not classified then Trump can talk about them in court, (or in public), without fear of violating the law.
They want to rush the documents to a grand jury so they can make a quick indictment whenever they choose.