Given that Col. Charles C. Marshall--Lee's military secretary and one of the founders of the Southern Historical Society and one of the leaders of the Lost Cause cabal after war--was its author, that ought not come as any surprise.
As a Maryland native from Harford County, Marshall is largely credited for numerous of Lee's writings, including the Final Order at Appomattox CH. You remind me that Marshall has not really been associated with Early's Lost Cause cabal leadership (like Pendleton, Jones etc) through writings condemning Longstreet's performance - at least he never spoke up against Early's overtly false charges against Longstreet. I think it finally Venable who years later repudiated the "Dawn Attack Order" fallacy.
The Gettysburg Report claimed that Lee didn't intend to deliver a battle so far from our base unless attacked - yet Lee was quoted famously about his plans to attack one early-arriving Union corps and driving one in on the next. The worst sophistry (IMO) involved the line: The result of this day's (Day 2) operations induced the belief that with proper concert of action, and with the increased support that the positions gained on the right would enable the artillery to render the assaulting columns, we should ultimately succeed, and it was accordingly determined to continue the attack."
Lee unilaterally determined to continue the attack. He foolishly ordered Longstreet to use McLaws and Hood's Divisions to attack again on Day 3 with no cover on their right. He had to have known that no "concert of action" at Culp's Hill was forthcoming after the early and mid-morning defeat of Ewell's corps. Lee seriously erred in ordering some of Hill's badly battered units into Pickett's Charge on Day 3 (many of whom had lost numerous key officers on Day 1). So, Lee's report indicated that Gen (Allegheny) Johnson was threatened on flank and rear on Culp's Hill on Day 3 "rendering it necessary for him to retire from his original position about 1pm."
Thus Lee knew there could be no concert of action in Pickett's Charge but he insisted on sending Pickett et al anyway.
And of course, Lee's letter to Davis of 7/31 reeks of Marshall's verbiage: No blame can be attached to the army for ITS failure to accomplish what was projected by me, nor should IT be censured for the unreasonable expectations of the PUBLIC. I am alone to blame, in PERHAPS expecting too much of its prowess & valour. It however in my opinion achieved under the guidance of the Most High a general success, though it did not win a victory".
Wow. Lee's only fault at Gettysburg was that he PERHAPS expected too much of his men - not that he made any error in ordering frontal assaults on Days 2 and 3. Any unreasonable expectation were those of the public - not Lee. Thus, maybe the army didn't have as much courage and valour as Lee thought. And of course, Lee doesn't acknowledge defeat, but credits the "Most High" for guiding the ANV to its "general success" at Gettysburg.
These reports are many light years away from "It was all my fault." Yet despite the written record, Longstreet has come to take most of the blame for Gettysburg and thus the war, while people like Shelby Foote make it into the Ken Burns TV series insisting Lee himself took the blame and said so to Jefferson Davis as well.