Plus he's lived for a century or however long 704's reaper tenure was.
I guesd they just meat that even tho he's lived for a long time, as a goblin who's life is very long (or immortal, idk) he is still toung and needs to learn more abt the world, get more experiences ad stuff like that. He also seemed quite kind ad innocent in a way, when he decided to take responsibility for some stuff but not say anything to anyone. So I get what they meanw but child was proably the closest thing they could compare him to.
He was in a coma/ hibernating most of that time though. Yama only woke up Gumsun once Hwan appeared around 20 years ago or so.
The little kid conversation was so unnecessary in that moment. So weird.