I'm not sure there is a rigid enough definition of IDE to say whether Emacs qualifies or not. I think it does by virtue of its extensibility, but I could definitely see a legit argument that it's merely an editor because it doesn't have a lot of the tooling of something more modern. I think what you consider to be an IDE (IntelliJ, VS, etc) is something that didn't exist until modern GUIs. Prior to that, terminal based things like Emacs (or LSE on VMS) were the closest analog.