From the link: "the total compute cost it would take to replicate the paper"
It's not Google's cost. Google's cost is of course entirely different. It's the cost for the author if he were to rent the resources to replicate the paper.
For Google, all of it is running at a "best effort" resource tier, grabbing available resources when not requested by higher priority jobs. It's effectively free resources (except electricity consumption). If any "more important" jobs with a higher priority comes in and asks for the resources, the paper-writers jobs will just be preempted.