TECO was just a DEC line editor, not entirely unlike ed/ex. The idea of TECO managing a display terminal (i.e. the environment in which emacs was written as a bunch of macros) came much later, and really only existed at MIT.

Glass TTY terminals were very new in the early 70's. TECO was primarily written to the "Knight TV" system, which was a fancy MMIO framebuffer array hooked up to a PDP-10 that would multiplex a bunch of displays and keyboards. There really was no "terminal" device per se, it was all software. There were other such devices (the MIT one was actually inspired by a similar system at Stanford), but nothing compatible enough to target an editor.

So basically if you were at MIT you'd understand TECO as the pinnacle of editting . But no one else could use it.

By the time Joy started writing vi, the idea of a "terminal" being a serial-connected device speaking a byte protocol with a handful of reasonable operations (position cursor, clear screen, etc...) had solidified. So a portable editor that would work with multiple devices was feasible. Emacs wouldn't get that for a few years yet.