That's not quite true. Cardiovascular risk is cumulative anyway
The significance of that depends on the individual.
Even softened water would be classified as low or very low sodium content on a dietary basis.
If somebody has health issues for which sodium of under 1.5g per day is indicated, then they shouldn't be drinking tap water anyway, be it hard or soft, since hard water can contain appreciable amounts of sodium as well. At this point, the person should be drinking demineralised water anyway.
The whole thing also depends on the original hardness of the water. The harder it was to begin with, the more sodium is added in the softenening process.
Probably of more importance in terms of cardiovascular risk is not presence of sodium, but absence of magnesium.
In terms of overall risk to health from tap water, I would be more concerned about presence of organic chemicals, pesticides, nitrates, fluorides (as in fluorides being toxic) and so on far before sodium content.
That may be, but its not because of any significant health risk to most people from sodium in softened water.
I can quite appreciate that softened tap water doesn't taste very nice. For that matter, hard doesn't either and so I don't drink either of them. However to suggest a health risk for most people is fanciful.