The temperature will be climbing all the time, until it reaches about 110 Celsius. This is when you should stop.
The procedure of making candy is to start with sugar syrup and then boil out some of the water, getting a solution which is fully saturated at its boiling temperature (which is above that of pure water) and becomes supersaturated when it cools down. This is how it works when candying fruit too.
So the word "simmer" is technically wrong in that recipe, as a simmer is usually around 85 Celsius. What they are trying to tell you is not to turn the heat all the way up and remove as soon as the solution reaches its final temperature, but to wait some time for the fruit to infuse in the hot sugar water (some recipes even involve repeated heating and cooling over a day or two before doing the final concentrating boil).
I think that it should be possible to do it sous vide, but you'll have to experiment with it. Hold it for longer time at a lower temperature, and for finishing, do the actual candying on a stovetop. The holding temperature will have to be above 70 Celsius so it will break down the cell walls and let the syrup penetrate the fruit. In fact, you may want to blanch it first, then leave in the bath for hours, and candy in the end.
And be aware, once you are on stovetop: the solution will take a long time to start getting over 100 at all, then also a long time to go from 100 to 101, but once it gets going, don't turn your back on it, it only needs seconds to gain another degree! Leave the thermometer in there all the time and remove as soon as it reaches your desired end temperature.
As for the exact desired end temperature, I've seen recipes calling for anywhere between 105 and 115. I don't know if it depends on the fruit type and/or thickness. Again, you'll have to experiment to find out which works out best for you.