I think there is a setting called ‘half power’ or ‘50% power’ which needs to be enabled in the advanced settings on the unit to overcome single sided power meters as it is basically just using the value it is given.
Had the Zumo for about a month now and have to say I think it is an amazing bit of kit. Totally changed my training coming from a dumb tyre on trainer whereby I would often blow before the end of a session by always going over the prescribed wattage and short power intervals were almost always impossible to do.
£405 with BC discount I would definitely recommend the unit.