Interesting, but very light on details about “cheaper than tapwater”. And the title is clickbait; the system they have is a small scale prototype and the developers only estimate that it would be cheaper than US tap water when scaled up. There is no further detail on costs in the article. Here is the extent of what it says:

the team estimates that the overall cost of running the system [when scaled up] would be cheaper than what it costs to produce tap water in the United States.

The article doesn’t provide any more info on the costs than the above quote. Dunno if the actual journal article provides more detail; I don’t have access. But I would need to see a lot more about how they produced that estimate and how uncertain the estimate is.