It states all the people of the planet were the Cetra, then at one point some of them stopped being Cetra, changing they ways of life and all that.
If Shinra's descendants arrived to an unpopulated planet, then that means they are the first Cetra.
Clearly, the intentions of the Cetra aren't the same ones as of Shinra's descendants.
And Hyper, who says it's wrong, or that he was making that up? True, errors like thinking Jenova was a Cetra are corrected later, but nothing is said about the Cetra in what Sephiroth was talking about.