I keep forgetting that this series is not about ponies
So even commoners have surnames in this world?
I guess so. Maybe they assumed he had one because he is well-dressed? Perhaps they are just expecting an occupational name like "smith" or a patronymic like "son of john".
It's not a big deal because irl commoners had last names for thousands of years in some parts of Asia, 2-10 centuries in various parts of Europe, since colonization for pretty much the rest of the world and since the 1870s in Japan. The architecture looks like ~14th century but it's fiction so we can't be sure.