When I was a kid we used hot water to defrost the windscreen regularly. A couple of hundred times over multiple winters. Temperatures around -20. No cracks or anything like that ever occurred.
When was that, out of curiosity? I seem to remember that the structure of the windshields (at least in Europe) change in the 70's from ones that would break with a crack, to the ones that would shatter in plenty of small pieces (due to the internal tension, and at least two layers)
Maybe they meant hot from the tap and not boiling, which will certainly crack the windshield.
I saw that in the 90s but people were definitely doing it in the 70s as well. I am not aware of any stories of cracks or shattered windscreens because of it, though I am sure that if you do that with boiling water and not-so-frozen windscreen often enough, you would get that result :)