As I always point out in these threads about mountain lions:
Cougars are not killing machines. And, probably due to the fact that their normal prey is four-legged and anatomically quite different, cougars appear to be lousy at taking down upright humans.
If you look at the data, the vast majority of cougar attacks end with various levels of injuries, but rarely death. Most adults attacked by cougars are able to fight them off, and even if not there are always signs of a struggle. And while it's definitely remarkable, a grown man desperate enough certainly has the body mass to smother and strength to strangle one.
One-jump take-downs of human prey from behind, breaking their neck and it's over, require the body mass and strength of the large cats. Man-eating tigers kill like that, cougars can't.