Queue the old adage ... "Emacs is a great operating system, what it needs is a good editor."
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.
> Emacs doesn't have a lot of the tooling of something more modern
Define "more modern"? Language servers, Git integration, refactoring tools, debugging? Emacs has all of those. Sure, VSCode and IntelliJ give you this stuff out of the box, but they can't match Emacs features like editing the same file in multiple ways at once (indirect buffers), instantly checking what any key does, or changing any behavior on the spot. What looks "outdated" about Emacs is actually its openness - while other IDEs hide everything behind pretty buttons, Emacs lets you see and change how everything works. You actually own your tools instead of just using them. In that way, Emacs isn't just modern - it's timeless.
I realize my definition is purely subjective in that I deal with Visual Studio and VS Code almost exclusively and that the VS Code team is instant that they make an editor and not IDE. For me an IDE has graphical tools for building UIs and other workflow items as well as language-specific compilers and tools built in. Like I said, subjective based on my experience (and my experience doesn't include Emacs).
Emacs is a text environment (which can also display images). It can also launch process and have IPC built in. So everything that works with text can be brought into emacs. And often, the only advantages of GUI is animation and aesthetics (to appeal to beginner). Text interfaces can be more productive.
Not only that - it supports PDFs (I annotate the books and papers in it), SVGs, variable fonts, emojis, even spreadsheets - yup, you can do Excel-like calculations; there are built-in browsers, etc. Besides, you can control music and video playback - useful when watching videos and taking notes, you can extract video transcripts, etc. etc. There even exists (albeit quite primitive) a video editor for Emacs.
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.