In his memo, McChrystal said he had heard Bush and Brownlee “might include comments about Cpl. Tillman’s heroism and his approved Silver Star medal in speeches currently being prepared, not knowing the specifics surrounding his death."
Go back to this post and read the speech that Bush gave two days after that memo was sent. One can logically assume that the White House Correspondents Association Dinner speech was one of those "...speeches currently being prepared..."
Notice it does not "include comments about Cpl. Tillman’s heroism [in any way that could be later called directly about his death] and his approved Silver Star medal."
It's evident that if not Bush, then the people writing his speech knew about the circumstances surrounding Pat Tillman's death.