Well, they're using the static, known density at 4 degrees Celsius to produce water with a known mass, and then test what the measurement of that mass comes to. Basically they're using a known mass of water as a calibration aid. But I don't know that this level of precision could compete with the efforts underway.

Exactly. They'd COMPARE it to the existing physical object used as the reference for the kg. Assuming water allover the world hasnt changed its properties over the last few centuries they'd know whether it became heavier or lighter.