Answer
I got confused a little bit, the correct typing was [String.t] and not List[String.t].
I will say however that:
lib/tip_calculator.ex: Undefined remote type Access:get/2 on line 0
Is not (to me) a very descriptive error message.
However, given the alternative (dialzyer) message is not much better:
lib/tip_calculator.ex:8:unknown_type
Unknown type: Access.get/2.
I will say however, the line number was an incredible help to find the issue.
With Gradient line 0 really threw me off.