The difference here is that with Typescript, you're not really measuring Typescript's performance, but whatever your output language is. If transpile to Javascript, you're measuring that, if you output Wasm, you measure that, etc, and the result isn't really dictated by Typescript.
Transpiling isn't the only possibility to run TypeScript code, it's just the way to do it right now. A long time ago interpreting was the most common way to run JavaScript, now it's to JIT it, but you can also compile it straight to platform byte code or transpile it to C if you really want. That you could transpile JavaScript to C doesn't mean all ways of doing it would be equally performant though.
Transpiling in itself also doesn't remove the possibility of producing more optimized code, especially if the source has more information about the types. The official TypeScript compiler doesn't really do any of that right now (e.g. it won't remove a branch about how to handle a variable if its type equals a number even if it has the type information to know it can't have been set to one). Heck, it doesn't even (natively, you can always bolt this on yourself) support producing minified transpiled code to improve runtime parsing! In both examples it's not because transpilation prevents optimization though, it's just not done (or possibly worthwhile if TS only ever targets JS runtimes as JS JIT is extraordinarily good these days).
