Also, the core editor experience of VS Code, the Monaco editor, predated Atom by several years. It was used from the very early days of the Azure Portal and also in IE's Developer Tools. It didn't become "VS Code" until sometime after Electron stabilized out from under Atom, but parts of it have existed for longer than Atom.

Slack does not aim to build a superset of features of Atom.

Electron could probably be described as "a big chunk" of Atom's codebase. The rest of VS Code has a lot of similar functionality to Atom, so calling it "a rewrite" doesn't feel that far off (except insofar as it was redesigned as well, but that's sort of semantics)