My headcanon is that some Ko-Matoran on Metru Nui predicted the coming of the Mata by looking at the stars 1000+ years before and recorded it prior to the Great Cataclysm. During 2005 the Toa Hordika/Metru found this information. Nuju was a Ko-Matoran who studied the future, so he could have made the prophesy, but he must’ve done it before he created Kopaka’s Toa stone as a Toa Metru.
Anyways this is a great topic and it’s brought up some interesting points.