I see three possibilities with why they would say there's no water there. One, there really isn't, which seems unlikely. Two, they're worried about liability and err on the side of caution, telling people there isn't any water. Or three, the water that is there wouldn't be safe to drink, even filtered. This could be due to runoff from farming, ranching, mining, etc.

The USFS says there's no water available in my local national forest, but I've never seen it completely dry. However, it is in a relatively flat area surrounded by ranch land, so it very well may be that they're worried about runoff. It could also be that it is indeed dry on rare occasion, and they're playing it safe.

Just a few things to consider. I would definitely carry or cache your water the first time you go there, just to be sure.