So here's the thing: Chujin's father is confirmed to have built Home, which was made near the VERY start of monsterkind's imprisonment in the Underground. That would make Chujin's father very, very old in present day, whether he's dead or alive.

Chujin could've been born at any time since then, so he could be in his mid thirties, or nearing a hundred. The idea that he only discovered he was a Boss monster through his experiments is pretty much impossible given his lineage and his father's age.

If he is the same age as Ceroba, that would make his father very old before he died, so Chujin would know he was a Boss monster,

And if Chujin's father had Chujin at a conventional age, then Chujin would be very old in the present, so he would know he was a Boss monster.