I’d have to see images of the cables to believe it. “Mainstream media” was not a term in use in 1945. It was coined 50 years later when Internet “alternative media” rose in popularity.
Also, you have MacArthur referring to the Japanese as “yellow bellied”, which implies cowardice. While their skin was yellow, I don’t think anyone (especially someone like MacArthur) could have mistaken them for cowards.
On the other hand, maybe we should listen to Honest Abe on this...