Well they didn't have the same ratios on the two. They used a constant speed motor of unknown rpms so we have no idea of whether it was done at a relevant speed. Depending on the specifications of the motor there are unknown levels of error in how much energy actually got to the crank. None of the three critical things (motor, Powertap, and magnetic resistance trainer) are known quantities and it is not know whether test conditions were specifically selected to give a bias to the belt. Further we don't know how well the chain was lubed and with what nor do we know the quality of the chain and sprockets.

Do you think that someone other than Carbon Drive hired them to do it?

Edit: I am not even saying that they are definitely inaccurate findings, but it certainly seems reasonable not to trust testing that was almost certainly commissioned by the manufacturer either directly or indirectly.