Remember that even on hot days not all of the traffic through the datacenter needs to be rerouted, and I'd imagine that a location selected for a datacenter like this was chosen for the infrequency of days that will require rerouting. Do you know how much it costs to cool a datacenter, and how much this will save? I don't, but Google probably does, and they probably wouldn't make a decision to do something like this without comparing the savings with the potential cost from decreased lifespan of computers running hot and losses due to downtime. I would also imagine that Google will be working to greatly increase stability during rerouting, given the comments from the end of TFA about other power saving uses, such as routing traffic to datacenters where it's night, meaning "free cooling" can be used since it's colder outside, and off-peak electricity rates are in effect.
I think the concept is interesting, and it makes me wonder if we'll see more datacenters built in areas of the world more conducive to projects like this in the future.