What regarding Sloan makes Bernstein panic?

Bernstein panicked because Sloan's story was initially based upon firsthand knowledge and suddenly it seemed to be based upon deduction rather than fact. This could ruin the entire credibility of the story.

Bernstein’s stomach began a slow dance of panic. He had been under the impression that Sloan had confirmed almost the whole story on the basis of firsthand knowledge, not deduction. Other sources had confirmed the underpinning of the story, true, but much of it rested primarily on what Sloan had said, and now he seemed to be backing off.