It's sort of moot as the goal of water treatment usually emphases removal of alkalinity. Many feel that there should be at least 50 mg/L present (125 ppm as CaCO3) though many excellent beers are made with less than half that. Citric acid does neutralize alkalinity (it is an acid) and there was a time when many home brewing recipes called for a tsp of it. The fact that it also chelates calcium might be advantageous in some cases and not in others. The other question I would have would be as to the fate of the chelate. It is somewhat soluble 800 mg/L which, with it's molar mass of about 500 implies 1.6 mmol and thus about an equal amount of calcium (64 mg/L) would stay in solution. It does have a sour salty taste which is why I don't think it is used in brewing much.
As for the particular product in question: it apparently does slow release of citric acid into the stream. Assuming that the release rate is slow enough to keep the water from tasting salty/sour I can't think that it would be terribly effective against very hard water. But of course I have no experience with this particular filter.
It is hard to imagine that it would be worse than a traditional salt softener from the brewing perspective. If, however, an RO system is in use or under consideration a traditional sodium based softener would be much better.
