It wasn’t exactly a rumor...the Spanish reported on it when they first arrived in what is now Mexico and immediately took steps to stop it. They reported that they walked through markets that had human arms and legs hanging from racks in the stalls. In fact, there are even drawings of this.
The reason it has become rumor is because it is a controversial subject in academe and among modern native peoples.
The reason it has become rumor is because it is a controversial subject in academe and among modern native peoples.