SvelteKit is not a “full” (depending on the definition what full means) stack solution either, and is commonly used with backends like Django or Rails. For example, it does not have its own ORM.
You’re still not getting what inertia is and keep asking dumb questions that make no sense. Please, check inertia docs. It has nothing to do with sveltekit/next/nuxt.
The whole point of it is it bridges the backend you know (Laravel, Rails, Django) with frontends (React, etc), without having to mess with JSON APIs.