The way I see it is Svelte treating the browser JS implementation as "machine code": while frameworks such as React or Vue are by definition a (runtime) layer on top of vanilla JS, Svelte _compiles_ your code to vanilla JS.

It's a very slight but interesting distinction.

Vanilla JS has long been used to refer to a lower-level JavaScript; ie not using any abstractions. Vanilla JS was often used to compare to jQuery. Of course jQuery is JavaScript.