As 1 liter of water weighs approximately 1 kg which is 1000 grams or 1,000,000 mg, one mg of something dissolved in it is 1 part per million. In the case of alkalinity, however, the meaning is somewhat different. In this case, 12.9 mg/L (the average reported alkalinity) means that the alkalinity is equivalent to the amount of alkalinity that would be found if one put 12.9 mg or calcium carbonate (limestone, chalk) into a liter of water and bubbled CO2 through it until the chalk was all dissolved and the water was at the reported pH.
An alkalinity of 100 mg/L does not mean that there are, in fact, 100 mg/L of calcium carbonate dissolved though that is one mechanism by which that level could be achieved. It does mean that there is the same amount of bicarbonate as there would be from 100 mg/L. The amount of bicarbonate ion can be calculated by dividing the reported mg/L by 50 (half the molecular weight of calcium (carbonate) and then multiplying by 61. Thus 100 mg/L alkalinity translates to about 122 mg/L bicarbonate. The actual amount is close to this but depends somewhat on the pH of the sample and the pH the analyst used as an end point when he measured the alkalinity. For alkalinity of 12.9 the approximation gives 15.7 mg/L bicarbonate. At pH 6.8 (lower end of the range in the report) the actual bicarbonate would be 15.8 (assuming titration to the "equivalence endpoint" was used in the alkalinity measurement0 though at the higher end (pH 9.2) it would be closer to 14.0. Not really at all significant at these levels.
This is a pretty impressive water system! Especially so is that they seem to be using UV for disinfection and only stabilize with chlorine for distribution (and don't use much of that). They do not say whether the chlorine is in the form of chloramine or straight chlorine. It's simple enough to determine which. Smell the water. Then let it stand overnight. If you still smell chlorine then they have used chloramine and you should add about 1/8 of a campden tablet to each 5 gallons treated. If you don't smell chlorine the next day then it was chlorine and you don't have to worry about it.
If you don't want to let it stand overnight, then you can use the campden tablet when you draw it.
This is fantastic water. You could brew Boh. Pils with it right out of the tap (or after letting it stand overnight) with no other treatment. For other more nominal beers an addition of 1 tsp calcium chloride will get the calcium up a bit and, if you want to increase the assertiveness of the hops, an equal amount of gypsum will do that (and increase the calcium as well). Try it without first, though, to see whether you like its effects or not.