Having worked in a large codebase to fix type errors in an evolving language, this is not true. There are plenty of errors.

What is true is that the errors you catch are most often benign and not obvious, user-visible bugs, for one reason or another. (If they were obvious, they would have been fixed sooner.) The code might be abandoned or rarely executed, or it's never called with that argument. Something like that.

Occasionally you do find a serious error, though.