Not necessarily, but even if you have to boil the water first, at least you still have water to use.

Hot water tanks are easy to purge and clean. Open the bottom taps, flush, shock, flush, test, repeat as necessary.

I suspect that when the main went, the contaimination wasn't immediate, and one could've stood a good chance of shutting off the input before the contamination came through. If one had a monitor/filter/treatment system on the front end, likely the contamination wouldn't have been an issue. But even without all the pre-emptive precautions, contaminated water could still be used in the household for some things. There too, it would depend on what the contamination consisted of.