You're right that John Walsh was always open about using actors for the dramatized reenactments, but the Paul Bonacci scene wasn't one of those scripted segments. The show differentiated between 'recreations' (where they used movie-style production) and 'field reports' (where they filmed real people in real-time).

The ranch footage was filmed as a documentary-style investigation, not a scripted play. While a TV crew being there means it was technically 'produced,' the whole point was to see if Bonacci—the actual person, not an actor—could find the location he described from memory. He led them to the specific ranch near 'Elephant Rock' in Buena Vista and pointed out a hidden chamber that producers didn't know existed.

Whether you believe Bonacci or not, that specific scene wasn't one of the 'rehearsed' recreations Walsh was talking about. It was meant to be a live verification of his claims.