No, it's not an OS by any means, as it doesn't provide a way to abstract computations (processes), doesn't provide support for file systems, etc.

That is a rather narrow (formal?) definition of OS. If instead you consider an OS to be (more generally) a collection of system-level software providing the interface between hardware and the userspace application, then in this case the sys code is the OS and the game code is the userspace. This definition is based on the idea that an OS manages resources and provides services - as opposed to the more rigid definition that an OS manages resources including processing time and storage space.