I see sugar mentioned in the baking instructions but not in the ingredient list.

I use this recipe with granulated sugar.
https://smittenkitchen.com/2021/04/classic-shortbread/

The same question was posed three years ago and went unanswered. The recipe was not posted elsewhere on the Internet so there's no way to check what the author's intentions were.

We could guess but I don't see the point. Shortbread is a traditional cookie. The world has tons of shortbread recipes, there is no compelling reason to use this one. Find another recipe from a reputable source that has been carefully proofed. There are many of them out there, including deadtrees cookbooks.

Feel free to add the optional zest and nutmeg if you want. But the basic shortbread recipe is three ingredients: flour, sugar, butter (and maybe a pinch of salt).

Best of luck.