Thank you all for the contributions. There seems to be a consensus around the "4 revenue rooms" thing, and I can't say for sure that the other pax in the 9 upstairsrooms weren't pass riders, or whatever... but there are reasons to be suspicious.
(1) I was assigned to room 17, close to the center of the car. Wouldn't the four rooms used logically be those closest to the revenue sleeper? In the event, all 9 rooms were occupied (one is transformed into a sort of office).
(2) The pax in those other 9 rooms did not, for the most part, "smell like" Amtrak employees. Several foreigners, mostly couples, children... Of course, could be that I just noticed those, and that they corresponded to the "revenue rooms." But it is certain that more than 4 cabins were occupied by passengers who did not appear to be associated with the railroad. The Australians opposite me had never been on a train before, for instance.
Still, who knows? I cannot be sure of what I say in all cases.
And I'm grateful for the input.
Note that I was in a high "bucket" room, after I accidentally canceled my res, and had to rebook. So it does not appear to be the case that the "high bucket" passengers are given some sort of preference in the regular car.
Rooms 17, 18, 19 and 20 are revenue. Only about 12 or so transition sleepers have a room 16 (it is not sold or booked for revenue or employees, because very few cars have that room). The other 30 or so cars have a shower/toilet where room 16 is. Room 15 is an "office" of sorts, though it really doesn't have much use except as storage. One sleeper had that room converted into a single-bed roomette with the idea that it might eventually become an attendant's room.
Rooms 21, 22, 23 and 24 are "BT" rooms, reserved for employee and company pass rider use. Note, also, that pass riders may include spouses/children of employees, and not necessarily employees themselves. Employees also include office staff and other people that "smell like" (not sure what that means?) non-Amtrak employees. Not every employee wears a uniform and BSes with the conductors for half the trip.
It's also possible that one or more of those rooms was used as an emergency substitute for another revenue sleeper room that was bad-ordered in another car.
But nonetheless, I've seen many pass riders in the BT rooms that, unless you actually spoke with them, you'd never know they were employees.