Is the reasoning that you then don't have to test what happens if you pass a parameter of the wrong type to a function?

The compiler guarantees it won't happen. Such a test would be a test of the compiler and in my opinion unnecessary. And in Haskell at least... impossible. You can't write the the test because IT won't compile!

I understand that. My question is if that is the kind of tests that people have in mind when they say that static typing requires fewer tests.

I can't speak for anyone else but I believe so yes.

Yes. When writing JS you have to explicitly do a lot more validation.