I guess given all the other issues, there wasn't really much point in the scriptwriter getting the numbers right either. But really - 1.3 billion tonnes??? The mass of the moon is 7.3x10^19 tonnes. If you added 1.3 billion tonnes, that would increase the mass of the moon by 0.000000002%. Really, no-one would notice the change in tides. That's only the mass of a small hill - roughly half a cubic kilometer of rock.