Learn Spanish (Spain): Su historia es pura fantasía: nada es verdad. - His story is pure fantasy: nothing is true.

Placing the adjective before the noun gives it emotional emphasis or highlights an inherent quality rather than just describing it. While fantasía pura would sound like an objective, literal categorization (like "pure, unadulterated fantasy"), pura fantasía acts as a subjective exaggeration, emphasizing that his account is absolute nonsense.

Why does the sentence use the noun verdad instead of the adjective verdadero (true)?

In English, we use the adjective "true" ("nothing is true"). However, Spanish heavily relies on the fixed phrase ser verdad (literally "to be truth") to express this idea. While nada es verdadero is grammatically correct, nada es verdad is by far the most natural, everyday way a native speaker would say this.

Could I use cuento instead of historia to say "story" here?

Yes, but it changes the tone slightly. Both can mean "story," but historia is a neutral word for an account of events. Cuento, on the other hand, often implies a deliberate lie, a flimsy excuse, or a literal fairy tale. Since the second half of the sentence confirms his story is entirely false, using cuento would fit perfectly if you wanted to sound a bit more sarcastic or accusatory!