Believable deep fakes, at least currently, cannot be created instantly. Blockchain guarantees whatever footage is on chain existed at the moment it was added and hasn't been tampered with since.
There are a lot of reputationally damaging images you could create for which it is not possible to determine the date when they were supposedly taken just from looking at them.
If you know that a specific cop is going to be at a specific location at a specific time, you could create the deepfake in advance and add it to the blockchain when they show up.
If you know that a specific cop is going to be at a specific location at a specific time, you could create the deepfake in advance and add it to the blockchain when they show up.