typescript thinks svelte is jsx?

create a tsconfig.json in root directory with this in it:

{
  "extends": "@tsconfig/svelte/tsconfig.json",

  "include": ["src/**/*", "src/node_modules"],
  "exclude": ["node_modules/*", "__sapper__/*", "public/*"],
}

https://svelte.dev/blog/svelte-and-typescript

assuming you use svelte-preprocess and lang=ts on the script tag