I have had that happen when I lived in Oklahoma, with my big water bottles that are delivered, we had them outside and one winter that was exceptionally cold, even my pool froze solid enough for my danes to walk on it, my big water bottles did that, it appeared fine and not frozen but the minute we picked it up to bring it in to put on the cooler zap solid ice.

I live in a northern zone and rely on heat tape to keep my water supply going. First, warm items cool off faster than cold items. So heating up an object to keep it from freezing actually speeds the freezing process. There are lots of heat tapes available that have a built in thermostat. You simply wrap them or tape them to the pipe and plug in. Up here in the frigid north we would put the heat tape on then insulate the pipe with a fiberglass pipe insulation. And yes, they can go through exterior walls and into floor cavities. Just make sure the thermostat is attached on the coldest part of the pipe (outside it sounds like) Heat tape I am not associated with product mentioned just using as an example. SCG

We're in MN in a hundred year old house we've lived in for nearly 30 years. We haven't done what you are contemplating, but I can see what the guy may be referring to. If you introduce another opening into the wall, even if you wrap those pipes in insulation, they would still be exposed to that very cold air since the rest of your wall space is basically an uninsulated void that could reach potentially be very cold IF an extreme cold snap came. Our kitchen plumbing is copper and on an exterior wall, but when we first moved in many years ago we insulated all of the walls in the house. Nonetheless, when we are anticipating really cold weather like twenty below, we open the doors in the sink cabinet so the warmer room air will get into that cabinet space and keep things a bit warmer than if the doors were closed. The problem comes if the water is not flowing through the pipes (think being on vacation). Some people around here if super cold weather is anticipated might also leave their faucet running at a slow rate all night to keep things moving. It is not as much a problem during the day because the sink would be constantly used during normal activities.

Our front yard (back in March) no snow yet, although tomorrow were supposed to have sleet and freezing rain.