I genuinely don't understand - how could Typescript provide a separate API on top of this? I usually do not expect Typescript to start generating code and APIs for me.

Also, Typescript is adding types on top of the JavaScript language, not the DOM API.

No they shouldn't have done. The Browser API Implementation doesn't exist in TypeScript. It exists in the browser. TypeScript shouldn't be fixing how the browser behaves.

Think about what is happening. When you build to the browser as a target, TSC basically transpiles the TypeScript code to JavaScript. The Browser APIs that Typescript provides are a bunch of type definitions. When it transpiles the code the only thing it can really do is check the types match up.