How does hiding water usage protect the facility exactly?

The big hyperscalers always knew that eventually the tide would turn against them (for example when local towns noticed that the DCs didn't actually need very many locals to run the facilities).

They carefully hid things so they could expand for as long as possible, knowing that having the necessary footprint for services, cloud, and AI would be key to winning the race. When I worked at Google, our leaders were always "proud" they had built such a huge fleet (originally for search + ads but rapidly growing to a wide range of products and research) without competitors knowing just how large, because they figured eventually AWS and MSFT would realize they had to build out massively to handle the coming loads.

So they hide these things to stave off complaints for as long as possible.