If you read further, however, you discover that the pianist had actually prepared to play both songs, but that their order had been switched, which is a significantly less dramatic story.
The pianist had less than a second to figure out that the orchestra was playing Rachmaninoff instead of Tchaikovsky, and make his big entrance. He didn't react quickly enough, and he missed his entrance. This may have cost him the prize. Afterwards, the organizers offered to let the pianist repeat his performance, but he was too upset to do so.
If it had been the other way around, and the pianist had been expecting Rachmaninoff but the orchestra had played Tchaikovsky first, it would not have been as serious. In Tchaikovsky's first piano concerto, there's a long introduction by the orchestra before the piano enters.
> 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.
I know that clickbait sells and all, but even "Wrong song played for pianist..." would be better and mostly accurate.
https://www.youtube.com/watch?v=UOUSFNgEy7c
Still an unpleasant situation, but not THAT bad.