I don't get the point. Does this imply that function signatures are to be trusted and bodies not? As I'm working on an implementation, I can't be sure I choose

I don't get the point. Does this imply that function signatures are to be trusted and bodies not? As I'm working on an implementation, I can't be sure I choose the correct input/output in functions, hence I can't decide that signatures are the final source of truth. I can get wrong both. Or it just means a "soft" precedence of signature over body? Sure, I read signatures first and deeper into function bodies later.