"I wasn't asking for proof," Scorpius retorted around a bite of chip. Proof would mean he believed there was a possibility that Hugo was telling the truth.
Asking for the story of how it happened wasn't the same as asking for proof. It was just asking for Hugo to elaborate on the story of his delusions. How they, in his barmy head, got together.
But why was he so curious? Why should he care? It didn't matter if none of it was real. It wasn't. Scorpius firmly stuck to his beliefs and drained the rest of his whiskey. He'd stop thinking about it. He'd stop thinking about Hugo and the way he kissed him and all the rest.