I think what you are missing is that your front fork is fairly rigid even under sag until you hit something big enough that the airspring breaks away and starts using up travel. We use low speed compression to help out but it's definitely not perfect. I was riding Harcourt this morning with all that loose over hard and you can feel the gravel in your hands through the front suspension. This constant vibration adds to your fatigue while riding. That's where buttercups do their thing, the layers of rubber in the system munch up that little chatter and the now the fork is always responding from gravel chatter to you big hits.
Back in the late 60s Stihl painted rubber vibration damping and was a huge advantage. You can find rubber vibration damping everywhere in manufacturing, cars, trucks, planes etc. The technology is proven and solid.
My 2 cents
I'm not disputing the technology just the claims that it does as much as is claimed in this application. The places where there are rubber connections to stop vibration are not already floating on a cushion of air (as far as I'm aware).
I think the issue with the vibration damping in this scenario must be to do with breakaway force/static friction (stiction). How else is the energy going to transfer in a jarring way directly from the lowers to the uppers (and then to your bars and hands). I'm questioning how much breakaway force is required to move the airspring and damper. Those are the only parts "affected" by the buttercups. The other parts of the fork that have potential to cause stiction are the stanchion seals and bushings, these parts are unaffected by buttercups.
For one it would be quite variable depending on how recently the fork was serviced. I know that the day after I do a lowers service my forks they feel great, and for some time afterwards. So when the stiction from the bushings and stanchion seals is reduced then maybe the buttercups might have some overall affect on the whole feel, but when the fork is not so fresh then buttercups are not going to do much. In a fresh fork maybe they do reduce the vibration by some amount but the vibration reaching the hands in that scenario is much less anyway because the whole system is performing optimally.
I could be totally off the mark, I'm no engineer just a keyboard warrior but I have a questioning mind and what bugs me is the unnecessary complication and hence extra cost in one of my hobbies. The mountain bike industry has made huge leaps in technology and we ride amazing bikes, but there are also bits on our bikes that are questionable or make very minor improvements (lets not start a conversation about hub widths and cranks spindles) and these makes our equipment more complicated and more expensive than it really needs to be. Manufacturers gotta sell more.
If your feel gravel in your hands try dropping your tyre pressure (3-5psi) and see if that helps. Tyre pressure is independent of what the fork is doing. Might do more than buttercups but might also have other side effects, there are trade-offs everywhere. Besides buttercups are only going to affect the first 4-5mm of lowers movement, bigger gravel means same "problems" as before.
My 3 dollars.
EDIT: "leaving you feeling fresh with more control"