Changing programming language neither reduces the need for test coverage nor does it magically increase coverage.

A type system changes the need for test coverage because it eliminates whole classes of bugs statically that would need an infinite amount of tests to eliminate dynamically.

That leaves an infinite amount logic bugs to be tested for. Types cannot fix interface misuse at integration and system level. So no, this does not reduce the need for testing.

Whether they reduce the need for testing overall is arguable. But what matters in this discussion is that types can guarantee memory safety, meaning that the cases that you forgot to test – and there will always be such cases, no matter how careful you are (just look at SQLite) – are less likely to be exploitable.

Types can only provide limited memory safety. There is a real need to deal with data structures that are so dynamic as to be essentially untyped. Granted, this usually happens in driver code for particularly interesting hardware, but it happens. Also, I have not yet seen a type system that is both memory safe and does not prohibit certain optimizations.