Before let/const, scopes could only be introduced at function level, so @babel/plugin-transform-block-scoping transpiles it using an extra function:

    var _loop = function (i) {
      a.push(() => i);
    };
    for (var i = 0; i < 3; i++) {
      _loop(i);
    }

The key is that the scoping happens for each iteration, not around the entire loop. That detail is nonobvious, given how many other languages have gotten it wrong, but I wouldn’t say it’s wild.

(If you’re curious how Babel deals with the more complicated cases of break/continue, labelled break/continue, and return, try it out at https://babeljs.io/repl.)