I'm sorry, I never worked in a data center nor lived near one, what's the obvious reason you couldn't dispute them? My only guess would be that Google really wanted to keep the amount of usage secret, but that'd only make sense if it was higher than expected not lower.
Also, they didn't want you to be able to guesstimate how many machines / computing power they had, for similar reasons.
People complain about new data center slurping up all local water and then neither Google nor representatives can say "No, actually we're not, here is the numbers", because Google doesn't want their competitors to guess how many machines they have in one of their data center? I'm sorry but that sounds very far-fetched, must be another reason parent wrote it was "obvious" they couldn't divulge how little/much water the place uses.
They published a lot of less specific info (PUE numbers, water efficiency, etc)
They were very sensitive about any site specific numbers.
It's probably more that they don't want to tell competitors anything they don't absolutely have to.