how can this not refer to emacs?

emacs -> screen -> tmux -> tmux integrated into <terminal app>

appears to be the path.

I guess the fork was "emacs before X" to "emacs after X but we retrofiited some ideas from X into the terminal" -> <the rest is history>

I advise to check Emacs history before its widespread adoption on UNIX.

Especially Zmacs and XEmacs, and how their features eventually found a way back into Emacs proper.