No O/S is installed on it/to it. It uses a simple ROM chip to make it available to the network.
It depends on whose definition of OS you go by then:
Operating System –noun Computers .
the collection of software that directs a computer's operations, controlling and scheduling the execution of other programs, and managing storage, input/output, and communication resources. Abbreviation: OS
To me if it can control I/O (network, storage controller) then it counts as an OS.
It is an OS in the most simplistic of forms, but it is an OS in my eyes.
Tape drives are an exception, because they're an extension of a system/server. Without a master server feeding data to the tapes the tape drives would just sit there. It just happens to be that that communication occurs over IP these days.
This argument is practically moot since Microsoft just killed off the only thing that made WHS truly useful.
All it will take is a stable Linux build with Windows compatible backup clients, and we'll see WHS replaced.
Me? I'll still run Acronis and backup over the network.