From the blog post, it seems that the 40% reduction is compared to the current utilization:
"by applying DeepMind’s machine learning to our own Google data centres, we’ve managed to reduce the amount of energy we use for cooling by up to 40 percent. [...] Given how sophisticated Google’s data centres are already, it’s a phenomenal step forward."
That "up to" renders the rest of the phrase meaningless. Was it 40% less for one second/minute/hour/day?
I assume, because they haven't actually given an impressive number without massive wiggle room, that it's because they are clever enough to know the real number is embarrassingly low, but would welcome corrections.
I worked at one of the datacenters where they rolled this out. It definitely wasn't all that impressive in terms of real energy savings, it would temporarily make our PUE drop, but we'd have to make up for it after the recommendations were secured in order to account for the temporarily relaxed set points. It was mostly a PR thing from my point of view.
"by applying DeepMind’s machine learning to our own Google data centres, we’ve managed to reduce the amount of energy we use for cooling by up to 40 percent. [...] Given how sophisticated Google’s data centres are already, it’s a phenomenal step forward."