Actually, the editor core is relatively easy. The hard part is adding all those 100s of little extra features you use every day. (Emacs ships with almost a million lines of extension code.)
Included in that million lines of code are two maybe three email clients, two IRC clients, org-mode (~100KLOC itself), tetris, and who knows what else.
I agree with your point, just not that the Emacs code base is a good reference of what's required to implement the numerous little text editing features people use every day.
