Google considers pretty much everything in hwops to be a trade secret. The point is to keep the specifics unknown as much as possible from competitors; the public at large's knowledge is just collateral damage in that fight.
This is right up there with being able to predict a competitor's total manufacturing capacity for the next year because you have hard numbers on their raw material purchase.
A competitor with that information might have made a rush on a supply-constrained component market or negotiated longer term pricing agreements.
So, given that you don't know what makes your company better then the competition, it's better to hide everything you can so you don't accidentally disclose useful information. God forbid competition discover your own strategic advantage before you do.
Power and water consumption number revelation goes directly to ceiling on maximum compute and relative compute efficiency. It's handing to every competitor on the planet (including China) "Hey, here's the raw situation on how far along we are on our total capacity to do anything."
... whether this should be a trade secret (since power and water are both shared resources with the surrounding community) is a reasonable question, but in the ecosystem where it is, Google having this data public and their competitors not is a problem.