I think the NTSB is being diplomatic. If you look at the maps, and bear in mind this was their chief pilot who had no doubt flown over Lake Berryessa dozens of times, there's no way he was mistaken. He didn't 'turn' into the canyon, he flew straight ahead into it, when the correct course was a sharp left turn. It would be impossible to mistake the canyon for the vast area of northern Berryessa.
I'm quite sure that he was demonstrating a canyon turn, deliberately choosing a canyon that was just wide enough, and I'm sure he'd done it plenty of times before. Both the images in the report, and some quick math, confirm that it would be easily possible. Unfortunately this time he pulled too hard (AoA goes up to 15°) and it ended badly.
There are a lot of very useful images here:
https://dms.ntsb.gov/pubdms/search/d...163&mkey=95127