"So if the down cannot absorb moisture, is it a safe assumption that there would be less moisture accumulation due to sweat evaporation while sleeping? Would the moisture simply get trapped between the fibers instead of absorbed into them? Or am I getting the concept wrong?"
Dead wrong. Down will absorb moisture and will stay that way until you add heat and some fluffing.
The issue when sleeping is that warm air rising from your body can hold a lot of moisture. It moves through the down until it hits the colder outer areas and condenses. For an overnighter, you will get by, but go for several days and it gets progressively wetter, clumping, losing its loft and insulating value, and getting heavier.
What do people do when hiking with down? Typically, they will allow it to dry as much as possible in the morning and spread it out in the sun at lunch time. Great idea if the dampness is slight and there is good direct sun.
Off I-90, 50 miles from Seattle this last July 4th, it was in the high 40's F at 4500' feet in the early afternoon, with heavy overcast and a big rain squall around 4PM. Humidity was high and the dew point was low. Hiking in the clouds, for real. Wake up on a cold morning and all your gear is covered with dew. Walk out through a brushy trail in the morning and you have to put on rain gear to stay dry: the dew looks like someone sprayed the brush with a hose minutes before you came down the trail.
Typical Western Washington rain isn't thundershowers. It rains lightly but constantly for hours, if not days. It might rain non-stop for a weekend and not accumulate 0.5". Add temps of 45-50F and humidity levels over 90%. Add constant overcast-- no direct sun for days. Add hiking steep switchbacks and trails with running and/or standing water, mud, and add a few stream crossings for dessert. There is nowhere for your perspiration to go-- you need a squeegee, not a towel. Get to camp, put up your shelter and shake out your bag to loft. Come back in an hour and it is cold and damp. Doing Leave No Trace, aka no fire, and you are living in a cold sauna with a mud floor. And you want to add a $400 sack of goose feathers to that mix? Insanity.
Drop the other side of the Cascades and the rainfall drops to 20" a year. Some of the stuff that misses the Olympics and Cascades makes it to Western Montana, but Get higher up in the Rockies and farther south and you have prime down country, along with the high deserts, and the Sierra. BUT, for anything in the upper left hand corner of the map of North America, down sucks. You can make it work, but synthetic is much safer and easier. And there is at least as much bad synthetic gear as there is down.
And when you mention Feathered Friends being in Seattle, that is not proof of anything. There is a big down market for high altitude climbing, trekking the Himalaya, the Rockies, the Alps, the Arctic and more. And then there is fashion. I've seen someone wearing a North Face Nuptse jacket on a downtown city street in 60F weather!