There's no scheduler inside Emacs. In contrast, there is something very much like a scheduler inside Firefox and inside Google Chrome so that more than one browser tab can make computational progress "simultaneously".
What Emacs does have (at least when it is running on a Unix-like OS rather than on Windows) is a way to use the underlying OS to create and manage child processes. I.e., a way to add processes to the OS's scheduler. But a terminal-emulation app that lets the user create and manage multiple terminal-emulation windows or tabs has the same capability, and no one calls it an OS with an OS.
Moreover, the programs `screen` and `tmux` also have that capability, and no one call them OSes within OSes.
People who like Emacs sometimes treat it like a generalized interface to information. But the combination of a Unix shell and a terminal-emulation app ("the Unix commandline") is often treated the same way, and no one calls it an OS within an OS. Ditto a simple web browser like Lynx.
EDIT: replaced "OS" with "OS within an OS" to try to make my argument less vulnerable to jacquesm's reply.