When I did my own test bookings, I pretended that there were at first just 1, then, 2 and finally 3 adults travelling, and the result was the same. No extra rooms became available to accommodate the extra passengers., that was how I did my testing to show almost all sleeper rooms were booked on that particular train.
I always travelled in a roomette for my earlier travels on Amtrak, and many of the roomettes and bedrooms were not occupied at all even back then.
My gut feeling, backed by my limited research, is that most trains now are running with most sleepers occupied, even at the high fares. It makes sense, folk in these Covid times prefer to keep away from strangers.