https://www.thoughtco.com/does-salt-in-coffee-reduce-bittern...
"The reason given is that adding the salt improves the flavor of the coffee. As it turns out, there is a chemical basis for this practice. The Na + ion diminishes bitterness by interfering with the transduction mechanism of that taste. The effect occurs below the level at which the salty taste would be registered."
[1]: https://www.sciencedaily.com/releases/2023/01/230130090347.h...
Then I'm looking at these ingredients and they actually look what they fortify milk with... Calcium carbonate, check, KOH, check [2] - Magnesium Hydroxide, I don't think they add that. I think Potassium Chloride might be an additive in feed for dairy cows but it's not an additive.
[2] https://www.vynova-group.com/blog/potassium-derivatives-dair...
(Brand doesn't seem to matter, I tried many, and I have no problems drinking just water, or tea)
I'm not brave enough to try it - my light roast pour overs are delectable without going this route, but apparently one of the best ways to reduce acidity.
They have a taste test in the end and claim that some of the versions taste better than their plain water. Not sure how much I'd believe this, especially as they don't specify their "plain coffee" and what the properties of the water were. In general softer water tends to be better for coffee taste than hard water.