ES6 is plain old JavaScript. I think the parent commenter's point was that even when you're writing plain JS, if you target browsers (and old browsers specifically) or e.g. a large range of Node.js versions, you often still need to transpile it. This naturally doesn't apply if you target a specific runtime version (e.g. a specific Node.js version).

That being said, the commenter above them could've also been referring to old old JavaScript where no transpilation is needed to reach practically every target environment.

ES6 implies transpiling, or polyfilling, IF you want to support most browsers still currently used.

It seemed to me that the author was describing the (somewhat common?) practice of writing client code directly in ES5 and perhaps polyfilling any missing features for any targeted browsers that required specific fills.

I, personally, find ES6 to be ugly and requires me to pretend all sorts of things that are not true for the targeted runtime. It's like a whole fake world, so I'll prefer to avoid it unless there's some obvious reason to use it, as in an existing front-end build toolchain and a spec to use ES6, etc. Greenfield? I guess Svelte and such will use it, but if Typescripting, then one will just write Typescript, right?