SvelteKit's offering is great. Much better than Sapper. For Vite the HMR on SvelteKit is better than Elder.js but SvelteKit doesn't offer Partial Hydration.

The main difference is Elder.js is designed for static sites and offers tools to help make building large static sites easier.

For instance, when it comes to building non-trivial static sites, there is a lot of data massaging that needs to happen and be in sync across the entire project. A good example is when reading from a headless CMS or generating a sitemap.

With Elder.js, you can massage this data once and add it where you need to via a hook and it will be available on all pages.

SvelteKit is less opinionated. It is apples and oranges.

edit: a word.