If such (refactored) method is being called from code that's not covered by tests
Well there's your problem :p
I agree that if code has no tests, it's more likely to be correct if it's in TypeScript than JS. But I don't consider that an acceptable bar, so if I'm doing your code review I'm going to ask for tests in either case. And once you have tests that verify the behavior of the code, you're not getting much incremental benefit from type checking.
Unit tests can only prove the presence of bugs; they can never prove the absence of bugs. Only a typechecker can do that (provided an appropriate implementation). E.g. here is an AVL tree that is verified balanced at the type level: https://sketch.sh/s/C7Afz38jtFRMTkkydeBgUA/
No unit tests needed to show that it is balanced :-)
If such (refactored) method is being called from code that's not covered by tests, the bug may not be discovered until it blows up in production. With static typing it would be caught at compile time.