Well, it's like saying static typing doesn't help you because both statically- and dynamically-typed systems need to do arithmetic. It doesn't really make sense

Well, it's like saying static typing doesn't help you because both statically- and dynamically-typed systems need to do arithmetic. It doesn't really make sense, and nobody is saying static-typing is helpful because it frees you from the shackles of a + b. The difference is that in a statically-typed system, you're validating data at your boundaries such that they comply with your nominal types that are then compiler-guaranteed to be internally consistent.