She really did say that, but not on TV. The witnesses had the privilege of deciding whether or not they were willing to be televised, and she declined (wisely). Costello didn't (unwisely, even though only his hands were shown). Needless to say, Chopper, her remark never appeared in the "official" transcript of the Kefauver hearings.
Those hearings were a real phe-nom in those days. There are many stories of how the power of TV absolutely stunned people in the early postwar years. That was one of them.