75% of the emissions in the report are from "scope 3" which is mostly embodied emissions of equipment. So the necessary sustainable power sources would be in China mainly, not ones serving Google's datacenters.
Maybe because they are trying to influence policy goals? Google is a large customer but it is still a teensy tiny fraction of American electricity consumption. I don't understand why the press has decided that it falls on the information industry alone to decarbonize the US (and global) power grid.