It's buried in the article, but TypeScript support is being worked on by one of the TypeScript team members in collaboration with the Svelte devs - https://github.com/sveltejs/svelte/issues/4518
The template language's restrictions compared to JavaScript/JSX-built views are part of Svelte's performance story. It's able to optimize things ahead of time that are impossible with dynamic code because of the constraints. Here's a couple tweets from the author about that -
The Svelte template language is quite small, and compositional features like slots and <svelte:component> are powerful. There are currently some unwanted edge-case restrictions as this comment points out - https://news.ycombinator.com/item?id=22541100