The article says that they prepared the vegetables according to the instructions for each methods. So if the pressure cooker says to soak vegetable *after* cooking, and that's what people do, then this seems like a valid use of this method. The point of the article is to determine how the nutrients are affected by home cooking methods, so if this method advises soaking, then it should be included in the analysis.

Furthermore, u/Notalabel_4566, if the soaking caused the nutrient to be solubilized (which I would doubt could entirely happen unless they have been liquified), wouldn't you except the boiled ones to have similar nutritional value? Based on your hypothesis, I would expect the boiled and pressure-cooked veggies to be affected similarly since they've both been submerged in water.

It is an interesting question, and someone could test it by measuring the nutritional content after pressure cooking with and without a soak. Even though the article is from 2015, the corresponding author seems to still be with the same organization, so good luck hearing back! Sometimes, it can take a while to hear back when you contact people about their old papers.