I use an IDE written in non-native code (IntelliJ) and I can have dozens of files open, while the IDE does error checking in the background across thousands of files, and it uses around 2GB of memory in that situation. 800MB for one file is insane.
This isn't just a contest between native and non-native code. It's a contest between efficient non-native code and crap non-native code.
To be fair they picked an example that really highlights a weakness in atom (parsing and syntax highlighting XML files), and this article is a bit older (at least in terms of atom and electron releases).
Right now I have Atom open with four different projects. Each project has anywhere between four and twelve files open. The project using the most memory is only 258MB, but the others are all close to 100mb.
Xcode is about as native as you can get (developed by the same company as the OS!), yet it becomes more sluggish with each new version. I think the real problem is product managers focusing too heavily on "shiny new features" and less on efficiency/stability, which is mystifying since these are all dev tools, after all.
I use Vim too by the way.