I get -27 F sometimes for a few weeks and it hasn't killed my top bar hives. Any hive, of course, can die. I think the important thing with a top bar hive is that the bees are at one end the the stores in front (or behind if you see it that way) of them so they can eat their way to one end and not leave stores behind them. When I have lost a horizontal hive it is usually because the cluster was in the middle and ate its way to one end leaving stores at the other end that they couldn't move to.
Just a note of interest; I have your book, 'The Practical Beekeeper'. Extremely interesting reading.