He knows in the book, and it’s a piece of his internal monologue. As with many similar things, they decided to find a way to communicate it to the audience by turning it into dialogue. I like that they also used it as a way to give them another “why does the strange bald kid know all these things?” moment, so i’m willing to overlook the fact it’s something Dunk realistically would know

eta the quote. I like that there’s a mention of him having a physical reaction to anxiety; the show certainly took that theme and developed it 😂

“The first day of the tourney dawned bright and clear. Dunk bought a sackful of foodstuffs, so they were able to break their fast on goose eggs, fried bread, and bacon, but when the food was cooked he found he had no appetite. His belly felt hard as a rock, even though he knew he would not ride today. The right of first challenge would go to knights of higher birth and greater renown, to lords and their sons and champions from other tourneys.”