Yes, it is true. When Pletnev was young he could prepare a piece he had never played before in a few hours and then play it as if he had practiced on it for years. Same with the Scarlatti, however he had played some of the sonatas on his Scarlatti CD in recital before (I have recordings of them).
If he could learn it in a few hours and play it as if he had practiced it for years, that wouldn't say much for his ability to think about a piece of music. He may have been able to learn the technical aspect of a piece incredibly quickly and perform it at a level that was very excellent, but I don't think anyone can play a musically challenging masterpiece at the highest level they're capable of after a short amount of time learning it. Surely no one can develop their final and best interpretation in a few hours.
What is the source of your information? There seems to be some disagreement on this thread about whether he had played some or most of the Scarlatti in previous recitals. If so, then the idea that he learned them on the plane is just extreme exaggeration.
No matter how well he played them on the record, IMO if he really learned them on the plane he was unfair to his listeners.
Pletnev is, by the way, one of my favorite pianists.
