I’m not sure the Inform IDE should have built-it support for external editors. For me, the IDE is arleady kind of a supercharged text editor.
In any case, with advanced text editors such as Atom, I think it’s the role of the text editor to add support for Inform through extensions, not the other way around.
Have you heard of the VS Code extension I wrote? Not only does it highlight Inform source, but it also allows to compile, play and release Inform projects directly within VS Code (and underlines errors when compiling).
Features are missing (jumping to a heading, creating projects and extensions, installing extensions, replaying previous commands, the skein, the documentation or the index), but it’s definitely possible to add them. (Well, it’s quite difficult for the index because Inform outputs whole HTML pages instead of only the useful data, but anyway.) In fact, it’s totally possible to fully replicate Inform IDE’s experience in VS Code.
I’m sure it’s possible to do the same with the Atom extension, if someone was willing to dedicate their time.