If you're only writing good tests, there's very few tests you wont write: eg, if I test that my function returns the string "Foo", I don't need to test that it returns a string.

And even in sound (er) type systems like Haskell or Elm, I'd (probably. There are ways...) have to write that test.

In JS with Flow or TS though, there's very few (good) unit tests you don't need to write anyway. If your tests are filled with "expect(foo()).toEqual(expect.any(string))", the team needs to be coached on writing proper unit tests regardless of types.

Though the topic that most teams don't know how to write effective unit tests and that type systems help those teams is a totally separate one.