I don't know about this particular case, but I have some (weak, anecdotal) evidence that at least sometimes the high-level structure is harder to remember robustly than the details.
I saw an amateur pianist sit down to play (a solo piano version of) Gershwin's "Rhapsody in Blue". He took from his pocket a bit of paper containing, not the actual score of the piece, but a lot of things of the form "Starts in Bb major", "Modulates to F major", "twiddly bit", and so on. Having put that in front of him, he proceeded to play the whole piece. Correctly, so far as I know.
Caveats:
1. n=1.
2. "Rhapsody in Blue" is probably an extreme example of a piece where this sort of thing would be useful. Here's a wonderful quotation from Leonard Bernstein: "The Rhapsody is not a composition at all. It's a string of separate paragraphs stuck together. The themes are terrific, inspired, God-given. I don't think there has been such an inspired melodist on this earth since Tchaikovsky. But if you want to speak of a composer, that's another matter. Your Rhapsody in Blue is not a real composition in the sense that whatever happens in it must seem inevitable. You can cut parts of it without affecting the whole. You can remove any of these stuck-together sections and the piece still goes on as bravely as before. It can be a five-minute piece or a twelve-minute piece. And in fact, all these things are being done to it every day. And it's still the Rhapsody in Blue."
3. This was years and years ago -- either 1987 or 1988 -- and my memory may be unreliable.
4. The amateur pianist in question was a teenager; maybe memory develops and/or degenerates with age in ways that would make this less relevant for adults.
5. Music versus poetry.
But it seems plausible to me. I have never memorized anything within three orders of magnitude of the length of the Odyssey or the Iliad, but e.g. when I was a stupid child I learned ~100 digits of pi, I learned them in 10-digit chunks, and right now I think I can remember what all the chunks were but I wouldn't want to place any bets on getting them in the right order. Of course digits of pi are more or less random and epic poems have narrative structure -- but making use of that narrative structure is exactly what GP is talking about doing.