I think your definition of an "operating system" may differ from the definition used by others such as myself.
UNIX was designed from the very beginning to be a "toolkit". When you installed a base UNIX system you got a bevy of small, powerful utilities that could be strung together into applications using shell scripts. It is at this level -- the "toolkit" level -- that I place the term "operating system".
Based on your description of an operating system, I can only guess that you're refering to the UNIX kernel as the operating system (i.e. the kernel is at the center, then the "shell" wraps around it, then there are apps and utilities that run in the shell, etc. I consider the operating system to be at the "shell" level).
If my definition of an operating system were to be used, the terminal actually would be a "window into the true operating system" as that would be the only way to gain access to the scriptable command line tools that make up the UNIX "toolkit".
Just my 2 shares of VA Linux stock...