Kinro and Ginro are guardians of the entrance of the village "the kingdom of science" now. When people from Tsukasa empire came to attack, they were the ones supposedly in charge of letting them in. But before they already were guards of the village, when they live in the world where supposedly no other human is alive.
Why Kinro and Ginro are guardians of the entrance of the village if in the world they leave no other humans are supposed to be alive?
While there may not have been (m)any people around, the guards were surely useful for keeping dangerous wild animals out of the area. After all, we know that there are lions in the region from early on in the series.
When Senku first arrives at the village, they say that outsiders are banned because the only ones out there are people who have been exiled from the village. Which suggests that there are at least a few people who have been kicked out at some point, although it's not clear who they are, when they were exiled, or if they (or their descendants) are still out there somewhere.
Manga Spoilers:
We eventually learn that the village is not the only human settlement left. Many centuries ago, their village was founded by a breakaway group that sailed to japan from the astronaut's original landing site. Soyuz also washed ashore on a boat as a baby. So, the villagers are vaguely aware that other humans might exist.
