That saying is what it is because of the fact that a grand jury proceeding is completely one-sided. At a grand jury proceeding, the defense pretty much just sits there, keeps quiet, and listens to the prosecution present its case. There is no chance for the defense to present evidence. The whole purpose of the grand jury is to make a determination as to whether the evidence that the prosecution has merits holding a full trial. Certainly, if the grand jury fails to indict, one can pretty much conclude that the defendant was innocent. An indictment, on the other hand, really doesn’t mean that much regarding the guilt or innocence of the defendant.
It’s my understanding that the defense team can submit a packet (information as to why to no bill) to the grand jury but that no defense lawyers can even be in the grand jury room.