To be fair, it was never a secret, it was made clear right from the design stage, with all the associated publicity, that it would have to have a back up propulsion system so that it could be moved around without needing a big team of rowers. It also has a bow thruster, because there was no safe way of docking it in the congested lower parts of the Thames without one. The photo below taken when it was carrying the Olympic flame clearly shows the bow thruster working to turn her around in a tight spot.
If you think about the logistics of moving this boat around when it wasn't taking part in the pageant, if they hadn't had motors they'd have needed to employ a big crew of rowers all the time. A friend who rowed in the pageant confirmed that the engine wasn't powering the barge for the actual event, in fact they had to keep asking the barge rowers to slow down a bit because under oar she was faster than the rest of the rowed fleet.
