From the article:

> he soon felt at ease in the Rachmaninoff, which he had prepared but did not expect to come until after the Tchaikovsky, in the final round of the competition [emphasis added]

It's a bit ambiguous, but it sounds like he had prepared to play it in another round of the competition, not in the same performance. There's a significant difference between having practiced a song and being fully (and mentally) prepared to play it in a specific performance.