Since this site is about sharing information, I would like to propose that you share with us how one would go about testing for matric potential hysteresis in flour. I'm sure it would go a long way to resolving this debate, and giving the credit you deserve for coming up with this idea.
Unfortunately, I've not the equipment nor the expertise in absorption science to test it. Nor do I claim with any confidence its effects are significant, only that they are plausibly pertinent. I only offer it in response to your request for an illustration of an alternative relevant physical principle, other than your evaporative principles, worthy of evaluation for potential effects in understanding sifted vs. non-sifted flour characteristics.
Instead of the example of matric potential hysteresis, I could also offer a less esoteric counter-example. Actually I am intuitively fond of the model that says the sifted flour should be drier because the sifted flour having absorbed more water has more successfully trapped that water in internal flour particle crevices, pores, and chemical bonds. This bound up water should be less available to produce surface stickiness (note: I am imagining surface stickiness as caused by "free water" - maybe a flaw in my model?)
But please let's not continue the theoretical back-and-forth. Instead let's work together to design interesting, creative experiments that give us useful data that enables us to actually observe what's going on to confirm or disprove whatever hypotheses we may have.