This was my first thought as well, but I do think it could still be useful, even if not all TypeScript can be run as pure JavaScript. There's a lot of use cases where some type hinting would be useful, but the full feature set of TypeScript is not necessary. Long term, I could see a world where these features effectively replace TypeScript, which could help simplify dev pipelines.
How would these features replace TypeScript? TypeScript is more than just a syntax as it also requires a compiler, config, IDE tooling, and typically additional plugins to make it compatible with testing tools and bundlers.
