A few years ago, the OU hired a bunch of Biologists to find out exactly what Quinoa was and they said it was some sort of berry or fruit.
They then send a delegation to Ecudador to the fields of where Quinoa is grown to see if Quinoa was even grown near wheat and it was not
After this massive reseach, They decided that it was not Kitnyot. Given the fact the OU is a recognizable name and would destroy their reputation if they declared something kosher when it was not and they did the actual painstaking research, their opinion must be given promimance