The point is not "let's just not write any tests". With a compiler that offers meaningful guarantees, you can write more worthwhile tests than "does this function always take/return integers".

If you’re passing or returning values of the wrong type, it’s going to blow up one of your tests. Asserting on a value implicitly asserts its type. Passing a value and not getting a runtime error for your trouble, pretty strongly indicates that it’s the right type.

Writing tests instead of utilizing the compiler is wasted time and effort. And it is one of the worst kinds of code duplication, because you are reimplementing all the type-checking, bounds-checking, etc. Usually badly, buggy and again and again. And since usually the test suite doesn't have tests for the tests, you will only notice if something breaks in the most inopportune occasion possible.

In unit testing for dynamically typed languages, very rarely do you make explicit type checks. The type checking naturally falls out of testing for correct behavior.

The "tests for the tests" is code coverage.