You're right, I never saw them or heard of them, despite growing up with chickens during that time. News, and even rumors, didn't travel as well back then. So the roo used the element of surprise on a man, I'll be the guy didn't turn his back on him again! Roos seldom attack head on, they may challenge, but they are stupid enough to come toward someone whose body language suggests they are aware and prepared, we are too big for them to take in a fair fight, so they resort to guerilla tactics. I used to kick that silkie roo every single day so he'd leave me alone to work in the pen. He's wait until he thought I wasn't looking and them attack, but I'd turn just in time and hit him full on with a steel toed boot. He never learned, we did this dance every day for years, the only time he hurt anyone was when they were not prepared for him. He was only a few pounds and could draw blood, so a large roo could definitely do even more damage, but he could never hurt an adult that was ready for him.
Border collies are great, very smart and obedient. This one was probably trained to tolerate chickens and essentially let the roo hurt him because he was trained to never fight back. If it was a dog with a string prey instinct like a husky or terrier, it would have ended much differently.
I'm just saying that rumors from the 70's can take on a larger than life aspect and get more and more fiction added as they are retold. We all like a story that is big and making it even bigger is nice. I don't doubt the basic facts, I just doubt that a chicken could be bred to be as fearsome as suggested.