As I understand it, a grand jury just needs to have a majority vote on whether they believe the person likely committed the crime or not. It's not a trial. They only have to prove probable cause, and nobody has to be interviewed for that if the documents speak for themselves (which they do).
Besides, if these grand juries are putting out sealed indictments, would there be a list of those they interviewed? I mean, seems counterproductive to keep the indictment sealed and let everyone know who the witnesses are. Isn't one of the reasons they seal indictments to protect witnesses?