Yeah, that does make sense to an extent, but "insulation" isn't the complete picture. Being a physicist, there are lots of fun things you can do with simple barriers (as opposed to the more traditional insulation) - and barriers can do wonders with heat retention and reflection.
Without something to hold the dead air, there won't be any as you mention. There can be dead air if you want there to be, you simply have to put it there. Take the pockets on the Clark NA model for example. I simply filled each of the pockets with a couple of large ziplock bags that I inflated with air. This created lots of dead air directly against my back. No insulation per se, but lots of dead air. This created a barrier with one side being warmer air from my body heat and the other side being the outside (below freezing, snowing and quite breezy). The result of this simple dead air barrier? I was quite cozy in the Clark.
The days I camped in the Clark were specifically to test the barrier idea to keep warm, and it worked fairly well. Without the inflated ziplock and drybacks in the pockets, there was no effective dead air (and certainly no insulation) - resulting in a very cold hammock. The pockets by themselves did not do much help stay warm except block breezes. Once they were filled with the airbags, I was able to stay warm using only an ultralight 3 season bag. I was pretty happy with that result. The next part of the experiment was to add reflectors to the barrier, but I didn't get around to it by the time I gave the Clark back to it's owner. The trapped/dead air barrier is not a new concept, lots of survival gear uses it - from a blow up vest to keep your torso warm to a blanket with enclosed air cylinders to make it much warmer than it's thinness would suggest.
The problem that many of these have is that they aren't breathable. If it's airtight, it simply won't breathe and that can be clammy, sweaty, condensing, etc. All bad things. It wasn't a problem with the Clark, I think because the pockets had gaps between them, as did my very high tech ziplock freezer bags. That seemed to be enough that it wasn't a major problem. I had frost on the netting and weather shield on top, but nothing too major.
Bottom line, I'd be fine heading out in winter with the Clark NA and a set of Glad freezer bags to hold some dead air. Much lighter than the heavy UQ that my partner was using, and he complained he was freezing (he's a cold sleeper though). I want to add a similar capability on my DIY hammock. It would be nice to be able to do it without adding airbags or insulation or something like that. Unfortunately, as you mention - there just isn't any dead air that way so I don't think that idea has legs. If I add a double bottom that can hold some airbags in place - I should be able to create a barrier to reduce/limit cold transmission/heat loss. If I need more warmth, then I add radiant reflectors to it. Super easy to do, but getting durable reflectors is a challenge.