Yes, but what about mutating objects/primitives in a way that causes all validations to pass but still be passing the wrong data?

It seems like using type checking as a reason to write fewer tests actually just creates nastier bugs in areas where your coverage is based on type validation.

What am I missing about checking type mutation !== mutation being a problem?

What is the advantage of strong types if your data structures are immutable? Is there any? I'm genuinely trying to understand if TS is just for cases where uncontrolled mutation is the norm and test coverage is poor.