TypeScript has at least 2 implementations: tsc and Deno, but I take your point and agree. That said, most languages probably have a JS runtime, so a basic implementation would be ts->js->interpreter. The ts->js is presumably pretty easy.
And they absolutely use TS for the std library, just not in the runtime itself
What exactly do you mean by this?
> And they absolutely use TS for the std library, just not in the runtime itself
Yes, internally. The code that actually makes up deno itself is just plain Javascript.