The libraries underneath will still allow nonsense at runtime
Only if you use a badly written library. Most libraries have types provided by DefinitelyTyped. Those who don’t are (in my experience) so tiny that you probably aren’t using them; or, if you really wanted, can check yourself.
In the end, if you encounter a bug, it’ still 99% of the time not a library’s fault, even if it’s written in plain JS.