Observable only supports dynamic imports, as the static import syntax is reserved for notebook modules (that is, cells imported from other notebooks).
Additionally you need to use a bundling service like skypack, jsdelivr or esm.sh. I recommend esm.sh as it lets you specify the dependencies or even create your own bundle on the fly, something you may need for the other question that you posted. Check out their docs for more details.
You can import BabylonJS with
BABYLON = (await import("https://esm.sh/babylonjs@6")).default