It may not be "accurate" in a naive technical sense but it's not disingenuous. KWh are fungible and electrons don't care who uses their voltage potential for work as long as it goes somewhere. Every KWh that Google produces with solar is a KWh the fossil fuel power plants don't produce CO2 for, which is the entire point: a net zero impact on CO2 emissions for one of the largest pieces of computer infrastructure in the world.