It's an operating system insofar as it's a layer of services on top of the "bare metal" that is the EVM. Through those services it allows the development of complex applications in the same way a normal OS does.
zOS takes this one step further, including not only libraries for SC development, but also aiming at defining interoperability standards, mechanics and economics for having independent contracts interact between them as independent actors (or processes, if you will) on a shared computing space that is the blockchain.
I think the key here in terms of semantics is that zOS is not just an Operating System, but rather an Operating System for the blockchain. And when we go there, we don't have many definitions available, but are rather waiting to be made.