This could be due to the fact that children are constantly running around Korean cities unattended at all hours of the night. Whether it's because their parents run some all night foodery and told them to go outside to play or because their parents are drunk in front of the 7-11 every night, it's common as fuck. Korea is relatively safe, so nobody really pays attention to kids that are alone.
Or his story could be bullshit. Which is also quite possible.