This is a very good question and concern. I wondered the same thing when I read that on the NW's site.
I don't recall where; but Waldig (Woody) had his explanation somewhere here about the failures. Something about the expansion/contraction co-efficient of two different metals. Block and IC? I'm not sure. Hopefully there aren't that many that this has actually happened to. I don't know if this has anything to do with it, but some of our cars sat for 2 years or more before being sold to an end user. The factory coolant has a rust inhibitor in it, so that's probably not a viable cause. Just throwing that out there for debate.