Do you label a button Submit or Save or Update or New or Create? That’s copywriting. Do you write three sentences or five? Do you explain with general examples or with details? Do you label content New or Updated or Recently Updated or Modified? All of this matters.
Good writing is good design. It’s a rare exception where words don’t accompany design. Icons with names, form fields with examples, buttons with labels, step by step instructions in a process, a clear explanation of your refund policy. These are all interface design.