I agree, they did make a strong statement at the end, maybe stronger than was warranted, but it was a valid scientific proposition, what they believe is the logical conclusion of their research. Yes, I think they presented reasoning to back that up, in the paper. They found two identical binding (integration) sites in two different cell types, which historically has only ever happened in contamination events. If true, this would seem to undermine XMRV in the human population, but a single study like this is not proof. Further research would have to confirm and extend their finding. They should have added a sentence at the end to state that fact. So they left a false impression with readers, overstated their conclusion, IMO. This is a very early finding. But if it turns out to be correct, that might shake things up a bit.