We need not worry unduly about the factual void at the center of the FBI director’s announcement on Friday that the bureau had found emails—perhaps thousands—“pertinent” in some unspecified way to its investigation into Hillary Clinton’s handling of classified emails while she was secretary of state.

True, we don’t know what is actually in the emails of Huma Abedin, Mrs. Clinton’s close aide, but we can nonetheless draw some conclusions about how FBI Director James Comey came to issue his Delphic notice to Congress, and what...