Unfortunately, this is subjective. For me, it's a combination of both. The other day, I interviewed a lady who'd just started off with her career, working on creating Oracle forms. She hated it. In the meantime, she'd started dabbling with Python. She didn't know what pip was, or how to install Python packages, but it was pretty clear she had a knack for writing code. With a bit of a push, she'd do wonders. So I hired her.

TL;DR - "Natural affinity" should include some level of skill/competence, plus that glint in the other person's eye when you talk about writing efficient, beautiful code.