Walter Lord does not state the source of his assertion, but regards the dates as significant:
May 31: Diversionary raid on Diego Suarez, Madagascar; June 1: Sydney; June 3: Aleutians; June 4-7 Battle of Midway.
There is no suggestion that the Sydney raid was nothing more than a diversion. It was clearly a serious attempt to damage the Allied forces. The suggestion is that the timing was tied in to an overall plan.