The article initially makes it sound like the pianist was caught completely off-guard when the band started playing a different song to what he had rehearsed.
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.
What makes this so dramatic is the fact that in the concerto they played first, the pianist is supposed to enter dramatically in the second measure.
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 met a guy once that told me that he never decided beforehand which song to play and that way would be less stressed. He was a pretty good pianist. More than 10 years later, I almost only perform original pieces or improvisational pieces and if I perform often enough, I find that I have way less anxiety with performing.
And it seems the piece was announced (first incorrectly, then correctly – or the other way round depending on the view) while the pianist and conductor were waiting backstage. So the pianist was aware of the different piece even before he stepped on stage:
The announcement was in Russian, but the pianist is Chinese. He would have caught the name of the pieces but he probably didn't understand the announcer saying he mixed them up.
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.