With boiling water underneath you won't get a temp higher than 212 at sea level. And some heat will be lost by the pan itself so I think the temp of the oil would be closer to 200-205 which isn't hot enough to decarboxylate the thca unless it is in dry air. I guess that's why my oils and baked goods never come out right.

Next time I do some cooking I'm going to put the materials in my toaster oven set to a little less than 200 with convection and toast it for about an hour before doing the extraction. This is low enough to keep the THC from vaporizing.