I actually do sometimes wear shorts at this temp, but usually I'm one of the only ones, and I only do it if I'm doing something relatively high intensity like walking a few miles and going over hills so I expect to get sweaty. At 13C in SF, if the sun isn't out, I usually see people wearing winter gear!
Based on the url I'm guessing the developer is Polish, which I would expect to get warm enough for 13C to be culturally considered pants weather, but what do I know.