Thanks. I ultimately found photos and videos that clarified this for me. The source of my confusion is that my circa 2003 Cane Creek threadless headset isn't like that at all.
On my current headset the principle is the same. I tighten the expander and then use the top cap to preload the bearing, but there's a difference.
My top cap *is* the bolt. It has a hex hole in the top of the cap. A narrower part of the underside of the cap extends down to the expander and threads to the *outside* of where the nut in the expander enters. It doesn't have a bolt through a hole that threads in down inside of the expander bolt. Turning the entire top cap raises the expander.
I had no idea that my configuration was unusual, and that something else had become the norm in the interim.
Now it all makes perfect sense.
So, if that shoulder-shaped top cap on the Carbonda/FSA is what bears the compression load, I'm guessing it's strong enough for the initial pre-load of the bearings, until everything gets locked into place by the stem bolts. That suggests that piece is not plastic. Does anyone here know?