True, you are exactly right. That’s what SG is showing.
That said, what they proved is that their treated down will generate the heat, but then it won’t saturate with the moisture that is baked/moved away from your body. Most down layers immediately lose all insulation when they get moist, which is why heavy wool has kept its place in military applications and even cold, wet hunting environments. What SG proved is that not only would their puff coat generate the heat, but it wouldn’t do what most down layers do when they get damp, which is loose insulation value. The moisture can move through the down and into the atmosphere instead of soaking into it and saturating it.
But you’re correct, the base layers are important. If you buy the treated down but then wear a cotton t-shirt underneath, you’re effed. You have to have your system set up properly from skin layer out.