Wiretaps do indeed require minimization—but generally only to the extent “consistent with the need of the United States to obtain, produce, and disseminate foreign intelligence information.” In some cases, when an intelligence agency issues a report based on a wiretap, minimization requires the issuing agency to substitute a generic reference in place of a U.S. person’s name—e.g., “Ambassador Kislyak said that he was looking forward to watching the Grammy Awards on television and that he was hoping that [U.S. Person] would win an award.”
But a U.S. person’s name can be used when it is necessary to understand the foreign intelligence information in the report, and no serious argument can be made that Flynn’s identity was not necessary to understand the intelligence significance of his call with Ambassador Kislyak. The call is foreign intelligence information mainly because it involves Flynn.
In a related context, here is what FISA’s legislative history says about minimization rules and substitution of generic identifiers for U.S. person names when those U.S. persons are U.S. government officials (this passage is quoted in the chapter on minimization in my book on national security investigations and prosecutions):
One example [of a situation in which a U.S. person’s name could be disseminated in an intelligence report] would be the identity of a person who is the incumbent of an office of the executive branch of the U.S. Government having significant responsibility for the conduct of U.S. defense or foreign policy, such as the Secretary of State or the State Department country desk officer. The identifiers of such persons would frequently satisfy the “necessary to understand” requirement, especially when such person is referred to in the communications of foreign officials.
Flynn was not the incumbent at the time of the call, but the logic still applies. In short, this is not a close call.