Unfortunately, enzymes do not “renature” once they have been heated to the point of thermal inactivation and later cooled. Enzyme denaturation can be likened to cooking an egg, since egg whites and enzymes are both proteins and denaturation is exactly what happens when eggs are cooked. This can be seen when the clear, liquid albumin of the egg irreversibly solidifies and turns opaque when cooked.
I think there are several things going on with this batch and brews like this in general. For starters, your mash temperature was really not that hot. According to Kunze’s Technology Brewing and Malting, alpha amylase remains active up to about 167 °F (75 °C) and begins to quickly lose ac-tivity at 176 °F (80 °C) when it denatures. So mashing at 160 °F (71 °C) is certainly not the end of the world, in fact Kunze cites the optimum temperature range for alpha amylase to be between 162–167 °F (72–75 °C), which is a bit higher than the average zeitgeist of today’s brewing collective. While 160 °F (71 °C) is too hot for beta amylase to survive for long (most sources indicate its denaturation temperature to be about 158 °F/70 °C), there will be enough activity with most of the highly enzymatic North American malts on the market these days to yield wort with normal fermentability.
Beyond the denaturation question, let’s consider what happens to wort after wort collection and boiling. Assume an infusion mash is used with a temperature of 154 °F (68 °C) and the wort is run to a kettle or holding vessel without a mash-off step. Also assume that the kettle or holding vessel is not heated until completely filled, not an uncommon scenario for many brewers who infusion mash. The wort in the kettle or holding vessel has a mixture of dextrins and active enzymes and wort fermentability can certainly change during this time period. This is especially true if the mash was short and the concentration of unfermentable dextrins is high at the beginning of this hold period; this is another example of Michaelis-Menten kinetics mentioned in the earlier question from Mark Sponer about sparging.
Now assume that the temperature in the kettle or holding vessel drops during this timeframe. Since the mash temperature in this hypothetical scenario was just a bit above the denaturation temperature of beta amylase and we have the possibility of a short mash, it is well within reason to expect some active beta amylase to end up in this container because enzyme denaturation takes time to occur, is influenced by enzyme concentration and is influenced by mash thickness and substrate concentration. So as the temperature drops below the denaturation temperature these remaining “native” enzymes (meaning that they are not denatured) remain active and continue producing maltose. This same logical argument about time and temperature is exactly the same reasoning that can be used to explain all sorts of things about brewing and food science, from pasteurization to staling.
There is a practical take-home message to this. The first is that brewing mistakes often are less catastrophic than they first may seem. In your case you thought you cooked your goose, so to say, and ended up with wort that was more fermentable than you expected. The second take-home message is that temperature measurement and control is pretty easy given the proper tools, and these are a good thermometer, reliable scale to weigh your malt and a reliable and accurate method of measuring water volume. As long as you know the temperature and weight of the two components of the mash (water and malt/grain) you can control your mash temperature and virtually eliminate losing sleep when the mash temperature is not on target.