In a PHEV, quoting the mileage with a XYZ L/100km is not correct, I would rather say misleading. For a PHEV, the correct quote should be either a full tank or with 1000km like BYD is quoting.
The point is when the battery is fully charged, you have a sufficient EV range to propel just on motors and the engine does not really kick in. The battery can propel upto lets say 50-80km ( in most normal PHEVs) without even needing the engine. So essentially after depleting the battery, you have already covered ~60kms and then rest with the engine upto 100kms, thus getting astonishing fuel figures.
MG has not a very efficient engine and the real mileage will then be to the rest of the 940kms if quoted with 1000kms mark where the car acts as a mild hybrid like Aqua/Prius instead of a PHEV. Overall, it will give near to 18km per litre over 1000kms which is good anyways.