The memo didn't mean that Katzenbach believed there was a conspiracy that had to be covered up. It meant that many--going by the first-day evidence and no evidence of other suspects appearing over the weekend--genuinely believed by late Sunday that Oswald was the lone assassin.
You can't really be that gullible. No one could've possibly known for certain within 48 hours of the assassination that no one else was involved.
The Katzenbach memo and all other actions taken by the Johnson administration were due to politics and national security concerns, not a good faith effort to find all the facts about what led up to JFKs murder.
The memo basically admits that Oswald's background is suspicious and invites conspiracy speculation. Duh.
Assuming that Katzenbach, Johnson, and other insiders knew about Oswald's trip to Mexico City weeks before the assassination, I don't buy that he was totally convinced that Oswald acted alone so soon after 11/22/63.
Even if Katzenbach assumed that all the shots were fired by Oswald only, it couldn't be ruled out within 48 hours that a guy who lived in the USSR and had recently traveled to the Cuban and Soviet embassies in Mexico had no accomplices.
Katzenbach, Johnson, and others were concerned about the politics, not the truth. This is confirmed by Johnson admitting years later that he didn't believe Oswald acted alone:
"Johnson expressed his belief that the assassination in Dallas had been part of a conspiracy. “I never believed that Oswald acted alone, although I can accept that he pulled the trigger.” Johnson said that when he had taken office he found that “we had been operating a damned Murder Inc. in the Caribbean.” A year or so before Kennedy’s death a CIA-backed assassination team had been picked up in Havana. Johnson speculated that Dallas had been a retaliation for this thwarted attempt, although he couldn’t prove it".https://www.theatlantic.com/magazine/archive/2013/08/lbj-oswald-wasnt-alone/309486/RFK, Gerald Ford, and Nixon all expressed similar sentiments in private. Johnson was the only one to say it publicly.
Then the Warren Commission followed the evidence that led them to the ingenious Single Bullet Theory (or Fact). Critics are jealous because they have nothing as clever (yet Occam's razoresque) and scientifically-sound that "solves" the assassination. Their lamebrain theories and goofball trajectory gyrations will always come up short.
There has never been a bullet in history that did what CE399 did and came out looking as clean.
Joe Rogan says it best: