I cannot possibly see any chemical reason this would be true. If it is “widely known” then it wouldn’t be too much trouble for you to dig up a source, right?
I mean, yes, if water has sodium, chloride, calcium, then it will less aggressively pick up sodium, chloride, and calcium. But that doesn’t have anything to do with whether it will bind iron or not. In fact the increased electrical conductivity should make it more corrosive to metal, not less.
Now that there are more authoritative people on this, I’ll throw out my “everyone knows x” for verification.
The water that normally comes out of the lab purification systems I’m familiar with is usually slightly acidic, pH 6 (plus or minus a bit). That much I’ve measured myself. I’ve heard this explained as the water being so unbuffered that the pH is determined by dissolved CO2 forming carbonic acid. Is this plausible?
The mechanism is quite real although not necessarily limited to purified water. Even tap water can acidify from CO2 absorption, though obviously it depends on the concentration and species of ion present.