Well, there is usually something on the boiler to regulate the feed water. BUT, if you happen to get a leak somewhere in the system, that will just keep feeding COLD water into the system.
And that's where the problem is.
Without seeing exactly what you have going on, it's hard to tell. But here is the scenario I think they are referring to.
You have a leak somewhere in the system. As water leaks out of the system, your feed valve lets new water in to keep the pressure constant. The problem is, you are letting cold water into the system. If your Cast Iron boiler sections are hot, and cold water gets introduced to them, there is a good chance they can crack.
The plumbers and inspector you spoke with may never have seen this before, but I have seen it 3 or 4 times in the 15 years I have been in plumbing sales.
Hope this helps some.