It’s not running TS directly, it’s just preconfigured to transpile TS to JS without the user having to bring extra tooling. Neat, but you’ll see the docs still recommend tsc for type checking at build.
Why incur the type-declaration overhead if they are not used after all?
Plus, for local dev, iteration and watch/rebuild is more important than failing with invalid types on every change. Sometimes it’s helpful to circle back to fix/update types after you’ve tried a couple approaches. (TS can still be finicky at times!) On top of that, your IDE should report type errors as you work anyways.
I would still prefer though that Bun did it for me, in a separate process perhaps, so I wouldn't need to configure a separate CI job, or manually enter the tsc-command. I read that Bun has its own test-runner too so why not its own type-checker too.
On Node.js I just edit the source-code then re-start the debugger on it, and edit it while in the debugger then rinse and repeat.
I use runtime assertions to catch errors in argument-types etc. as needed.