It is serious but in hyperbole. The water is probably near freezing, but not below. Frozen pipes are a big problem in cold regions, they expand and burst, so measures are taken to insulate and keep the pipes warm. Depending on the piping though, sometimes the only resort is to completely drain the pipes during winter and not have any running water for that part of the year. (Common in cottages rather than real homes)

I believe they were exaggerating a bit. Water pipes are typically buried deep enough in the ground that the water gets very cold but not frozen. The pressure in water lines isn't high enough to really drop the freezing point of the water to an appreciable amount.

Groundwater tends to be around the local air temperature averaged across the whole year. It is considerably colder in northern climates.

Not literally supercooled though, pipes aren’t that pristine, they will happily freeze.