Sure, I think the rebuttal is that while your numbers are correct (and possibly even more in your favor than you may have guessed) they aren't particularly relevant.

First, the numbers. My searching says that for some particular lake in Nebraska, "Average annual gross lake evaporation ranges from about 48 inches to 54 inches". Let's call it 4 feet per year, which means 4 acre-feet of water evaporates per acre per year. Trusting some numbers someone gave in a comment earlier, the data center in question used about 40 acre-feet per year. So 10 acres of lake would evaporate about the same rate as the data center. At a very rough guess, there are something like 200,000 acres of non-seasonal lakes in Nebraska, so this is indeed a tiny percentage.

Next, the relevance. The lakes are more or less in a steady state. But if you were actually to double the evaporation rate for a 10 acre lake, you might quickly end up with a muddy pit instead of a lake. The fish would be gone, and the users of that lake would have good reason to complain. If lake was 10 times larger, so that you were only increasing the evaporation by 10%, over time you'd still probably have a major effect on the shoreline and any infrastructure there. What used to be a useable pier might look over a mud flat rather than the water, and launching a boat would be impossible until you built a new one.

But the real issue is that water use isn't actually fungible. Water continuing to evaporate from a lake at the rate it always has definitionally doesn't make people's life worse than it was. But suddenly pumping of the same amount of groundwater might be really bad for the neighbors who previously were using the same water to irrigate their fields. And trying to double the output of a small municipal water system might (at least in the short term) make things much harder for all the existing users on the system. This doesn't necessarily mean it shouldn't be done, but it's a harm that should be considered.

That's my good faith attempt to answer you. In return, I have a quick research question that I think is essential to understanding whether or not data center water usage is a big deal or not, and that I haven't seen mentioned yet in the comments: Where is this water for these new data centers coming from? Is it from new wells they drill? Existing irrigation wells they buy and repurpose? From the local municipal water supply? Rainwater collection? Diverting a nearby river?

I feel like there's no way to answer the question of whether data center water usage matters without considering the actual local impact, and no way of determining the impact without considering the specifics of the source. But somehow I haven't seen any mention of this on either side of the debate.