This is true. It has been proven by world-class athletes time and time again. When you read about their diets, you see that all the protein they need (no more than 30 gr/day!) they get from nuts and soy beans.
Bodybuilders are a special case, so they can get away with eating some more protein (maybe 10gr/day more), and especially more soy beans to promote pec development. But beware, there have been many bodybuilders who have died of exploded kidneys because they were eating 50-60 gr/day of protein; these deaths have consistently been covered up by the mainstream magazines and the supplement industry.
It is sad that when the truth is brought up on a forum such as this, it is ridiculed in such a way.