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.
> which can also display images
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.