Classic brainwashing begins by washing the old sense of self, and adding a new one. In basic training, it works by forcing individuals to conform as a team under intense pressure. In Patti Hurst's case, it was days locked in a closet going to the bathroom on herself. I don't doubt this poor girl went through something similar.
After that, the mind is open to receive whatever the trainer wants.
I have no doubt that these guys did something similar, whether they understood what they were doing or not.
I'm not an expert, but I think that "Stockholm Syndrome" and classic brainwashing are one and the same. In one case the individuals merely discover a way to work together by instinct, and in the other it's planned ahead. But the symptoms on the victim are identical.
We just don't encounter it much these days.