I've read several of Manchester's works and admire them very much, but
I have not read Goodby Darkness. The guess that I make is that since
this work is a memoir he didn't feel the need to fact check as much as
he might do for a conventional history. To the soldier/marine in the
mud, war is a different set of realities which color his view of the
conflict. Manchester's recollection of the war was shaped, in part, by
bad information and propaganda and that's part his war.