Why aren’t the data centres just honest about their water usage if it’s seemingly so low?

They are, and people just accuse them of lying!

Facts & reason do nothing to a moral panic.

Transparency is not the same thing as honesty.

Companies might want to avoid this being used as a side channel to infer the amount of their compute.

Companies might want to avoid setting a precedent where activists can demand such information.

Companies might want to avoid the effort of publishing this data, or the risk of publishing incorrect data.

Because the marketing teams don't do numbers and it's a high risk strategy

I don't understand your question. I'm commenting on the figures we know from this report. I don't have to work out the motives behind them to evaluate whether they're material (they are not).