I somehow missed this stimulating article during a busy May.
Although the fact that this is a comparison between apples and oranges has been acknowledged, some aspects of the differences haven't been highlighted (or have been underplayed when representing the results.)
Quilting - may reduce flappage and windage.
Insulation with quilting through multiple layers is stiffer, resisting flapping that increases convective heat transfer and pumps air in and out of the garment. Winds where this issue would pose a serious problem may not have been felt by Dave during his testing.
Flatter lying surfaces - greater contact area with surface of garments beneath.
In the case of a wet t-shirt drying out under these two garments, I wouldn't be surprised if this was the main factor in the Xenon out-performing the UL Down Hoody.
(At a microscopic level these highly calendered, slick low-denier fabrics have a lower angle of contact with water than their lumpier surfaced, higher-denier but same-substrate brethren - the mere fact of a bumpier surface actually repels water droplets - that's how the lotus leaf effect works.)
The intricacies of the fabrics - hygroscopicity and air permeability.
The fabrics here superficially look to be very similar, but I've recently seen a study where a range of Nylon 6s were compared and their moisture regain varied from a fraction above 1% to a fraction under 10%. Add in a potentially greater variation in air permeability between the fabrics, and insulation aside, the variation in dry times for garments consisting of twin layers of these fabrics may conceivably vary enormously.
(Same branded membrane, but with a different face-fabric and you can see maybe about a 60% difference in breathability scores in RET and cup tests. There's huge variation and people just wrongly assume there isn't.)
And all of this is before we're really making a like-for-like comparison between the insulations; e.g. conductive heat transfer under compression, compressibility (in the wet, the dry, oil-soiled...) etc. That's another long list for another time.
Food for thought...