The title is a bit click-baity since I don't think anyone would think Observable would run javascript as if it were in editable <script> tags in a html document. But this is what most of the points try to explain: that there is a framework around the js it runs (so errors don't leak to other blocks (cells)), some types of imports are not supported since they are not needed, etc.
Not saying Observable isn't great, just pointing out that it is javascript - other languages have playgrounds as well.
JavaScript is the runtime executing it yes... but if you write C code and transpile it using Wasm you are confusing people if you say that what you wrote initially is not C code; so for the sake of comprehension (what language was created for) it is indeed "not JavaScript" but a new word we can agree to use to make it clear what plataform we are talking about: Observable.
Not saying Observable isn't great, just pointing out that it is javascript - other languages have playgrounds as well.