It has already been established that Dylan incorporated expressions, phrases and entire sentences from other authors in the book. A little research reveals that the same practice underlies parts of the MacLeish conversation and is responsible for the misattribution of certain statements to MacLeish. In this case, the source is the preliminary material to the Complete Poems of Carl Sandburg, (2002). This book has an introduction by Archibald MacLeish, and a preface (called "Notes for a Preface") by Sandburg himself. The remark about West Point comes in the Sandburg preface, not the MacLeish introduction:
Sandburg, p. xxx:
"At twenty-one I [Sandburg] went to West Point, being a classmate of Douglas MacArthur and Ulysses S. Grant III ..."
Chronicles, p. 112:
"MacLeish had been a classmate of Douglas MacArthur at West Point ..."
Confirmation of Dylan's borrowing comes from elsewhere in Sandburg's preface, such as this remark:
Sandburg, p. xxx:
"A few masterpieces last across the years. ... Perhaps no wrong is done and no temple of human justice violated in pointing out that each authentic poet makes a style of his own. ... I have forgotten the meaning of twenty or thirty of my poems written thirty or forty years ago."
Chronicles, p. 113:
"He said that he'd forgotten the meaning of a lot of his earlier poems and that an authentic poet makes a style of his own, a few masterpieces last across the years."
Sandburg (Preface, p. xxviii) also alludes to "Michelangelo saying in 1509, 'I have no friends of any kind and I do not want any,' and forty years later writing, 'I am always alone and I speak to no one.' " This too is picked up in Chronicles: "He also talked about Michelangelo, said that Michelangelo had no friends of any kind and didn't want any, spoke to no one" (p. 112).
Another possible allusion lies on p. xxvii of Sandburg's preface, where he says (italics added): "A well done world history of poetry would tell us of the beginnings and the continuing tradition of blank verse, rhymed verse, ballads, ballades, sonnets, triolets, rondeaus, villanelles, the sestina, the pantoum, the hokku; also odes, elegiacs, idylls, lyrics, hymns, quatrains, couplets, ditties, limericks, and all the other forms," and Dylan quotes MacLeish to the same effect: "Archie spoke about blank verse, rhyme verse, elegiacs, ballads, limericks and sonnets" (Chronicles, p. 112).
One final parallel: Sandburg writes of Stephen VIncent Benet (p. xxvi): "He knew the distinction between pure art and propaganda in the written or spoken word." Dylan says of MacLeish: "He also told me that there's a difference between art and propaganda and he told me the difference between the effects" (p. 112).
However, Dylan also attributes to MacLeish some expressions found in MacLeish's introduction to Sandburg, such as on p. xx, where the poet alludes to "... the comparative dimensions of ... Sappho and Sophocles, of Dante and Donne," which Dylan turns into this: "He asked me if I had read Sappho or Socrates. I said, nope, that I hadn't, and then he asked me the same about Dante and Donne." (Note the miscopying of "Socrates" for "Sophocles.") (I owe this observation to Scott Warmuth.)
It is now apparent what happened. Dylan, in the course of concocting (or reconstructing) a conversation with MacLeish, pulled from his shelves a copy of Sandburg's Complete Poems, Introduction by Archibald MacLeish, to get some ideas. However, he confused MacLeish's short introduction with Sandburg's long preface, and as a result wound up making MacLeish say a number of things that actually were said by Sandburg.
No doubt many Dylan-worshippers will now argue that this confusion is a sign of Dylan's genius. I think Bob just got his sources mixed up. It happens, especially when you're "borrowing" a lot from other writers.