That is a dumb way to describe the water quality needed for a recipe. But, I would say that they may be referring to hardness as CaCO3. If you assume that all of that hardness is due to calcium, the calcium concentration is 40 ppm. That is just barely adequate for normal brewing conditions, but you can brew with it.
Now if the recipe had called out a particular Residual Alkalinity for the brewing water, that could be OK and would be more descriptive. Unfortunately, even that description is not definite and can be produced with a wide range of hardness and alkalinity. A recommendation for RA of your brewing water is still kind of stupid, but it would more likely lead to an appropriate mash pH. To illustrate how dumb this is, look at Pilsen and Burton water. Both have RA's of approximately zero. But the level of mineralization will greatly affect the finished beers. Make an IPA with Pilsen water and its nowhere near as bold as expected. Make a Pilsner with Burton water and you wouldn't recognize it.
A wiser recipe description for recommended water conditions would be to provide approximate concentrations for each of the 6 major ions we are interested in for brewing.