It's exculpatory if the prosecution used a video of him doing one thing, said that's all he did in there, and used it to get him to plea out. It's exculpatory if the judge used presented video of him doing one thing, and made that part of the sentencing time, as if he was only doing that one thing. The video's withheld showed him doing another thing, which was never given to the defense, which could prove he wasnt ALWAYS doing the one thing.
The thing in this case is his interactions with the police and his general demeanor.