Actually, there were many clues listed, but they didn't directly connect them to "slaves" as such. The rough timber used in the bedding, the amphorae under the bed, a chamber pot, and lots of evidence suggesting the room was used for storage in addition to sleeping. These all point towards someone of much lower class (slaves of some kind) than the owner of the villa.
Nominally free lower classes would have lived in an entirely different part of town. We know quite a lot about the society in question from written sources, this is far from blank slate archaeology (where everything we find seems to default to "used for some ceremonial purpose")
I assume that in 79 A.D. the things you list, like beds no matter the timber roughness, were a luxury. Like, imagine living in a stone house? In a room with decorated walls (looks like carved out patterns on them).
Surely the scrappiest room in the villa was for servants, some distant poor relative or rented out to students or what not. I just don't buy that the inhabitants for sure were slaves.
Slaves, plebians, and patricians were very distinct social classes. There really wasn't such a thing as a free servant, with the occasional exception of some freedmen. Roman villas had two separate living areas, one for the owner's family and one for the slaves. We know enough about Roman architecture to identify this as the pars rustica, the slaves' quarters.
I assume that they have compared these accommodations to others already found and see a noticeable difference in the quality of the furnishings and other components.
Slaves made up 30-40% of the population so it not unreasonable to conclude that the shabby accommodations would be intended for slaves.
That's how all beds were made prior to the advent of box springs (or modern mattresses, which don't require a separate box spring). Very old beds still have the hooks necessary to hold up the ropes that supported the mattress.
I've seen medieval beds in the UK that were a wooden 'tray' (on legs, or built into a wall) and the guides said a hessian bag was filled with straw (dried stems of wheat/corn/barley; straw and hay are often used interchangeably though, so it might have been dried wild grasses) for bedding.
I assumed poor people slept on the floor with whatever coverings they could muster. Suspended beds seem very decadent.
The stretched fabric, like canvas, over a rectangular frame design seems pretty common and would have surely been a more natural early bed. Similarly hammocks - which I thought were common on ships (at least in the later middle ages).
Guess I've discovered I'd like to read a history of beds!
