How to remove pedal bolt with round, smooth head
I expect that when you try to turn the nut you'll rind that the round bolt head is eccentric on the bolt. Side on it looks like the sketch below, and means the bolt won't rotate once it's in the hole.



This is quite old school, and they're very easy to manufacture with only fairly basic hand tools. Imagine trying to cut a hexagonal hole into a pedal using only a drill and file.