I think it was important to him since otherwise leftPad probably wouldn't fit in a tweet so nicely..

Umm isn't babel-runtime an actual third party language runtime dependency (like polyfills and shared code for stuff like async) ?

I wouldn't really call it a dependency since it gets inlined with the shipped code.

Indeed, it is the only post-build babel dependency required if you want things like you say: async/await. The rest are the babel dependencies are build tools.