A question about a recipe: Rhubarb Curd Shortbread

I have a question about step 3 on the recipe "Rhubarb Curd Shortbread" from Rivka. It says:

"Add a couple inches of water to the pot of a double boiler and set over medium heat. Put egg yolks, butter, remaining sugar, lemon zest, and lemon juice in the boil of the double boiler and whisk to combine. When sugar has dissolved completely, add the rhubarb puree by the spoonful, to temper the eggs. When all rhubarb has been added, set bowl over pot; the water should be simmering. Continue stirring the rhubarb mixture; after about 5 minutes, the mixture will be warm and slightly thickened. At this point, remove from heat. Press through a strainer -- this will give your curd that smooth, pudding-like texture."

In the part prior to adding the puree, are the ingredients in the boil over the heat or is the boil off of the heat?