Exactly. The fact that the the mosaic was truly 2,000 years old does not make it "true" but does make it archaeologically interesting. Imagine if 2,000 years from now archaeologists discovered the film collection of Oliver Stone and Michael Moore. I am sure it would be of interest, but not a representation of fact.
As I understand it, the Gnostic spiritual movement predates Christ. Jesus just gave them a more current vehicle to spread their word. They didn't acknowledge the human flesh quality of Jesus. The Orthodoxy at that time felt that belief short circuited their teachings especially about the Eucharist.