It is certainly true, as have bee stated, that the act of production of documents has been found to be testimonial in nature. However, the claim is not absolute and can be validly asserted. The only way to review the assertion would be as part of a contempt proceeding. It would depend if the assertion is closer to the 1976 case of Fisher v. United States or 2000 case of United States v. Hubbell.
The act of production doctrine was largely defined in Fisher where it was rejected as part of an IRS case seeking documents from two taxpayers through a demand to their accountants/lawyers. The documents were already known to the IRS and the securing of the documents was viewed as a “foregone conclusion.” It is not enough that the documents are incriminating. They must be compelled as a testimonial matter in their production.
Flynn would argue that the issue comes closer to Hubbell in the Whitewater investigation by Independent Counsel Ken Starr. The grand jury subpoena duces tecum in that case was sweeping and poorly defined. He was given immunity to produce the documents under 18 U.S.C. §§ 6002-6003. The Supreme Court found the production was testimonial because the demand did not meet the required demonstration of “reasonable particularity” for the documents.
Flynn could be challenged in his assertion by saying that this is not the same type of “fishing expedition” as Hubbell but rather comes closer to the “foregone conclusion” of Fisher.
Thus, Congress could move for contempt and argue Fisher against Flynn.
Flynn’s lawyers were correct in taking this approach for their client but the ultimate success depends on the basis for the invocation.