Why should he have to do that?
ianal but absent some nda seems like it's probably legal for him to do it.
it just seems like kind of a dick move. i could mentor new engineers, wait until someone told me about a really cool idea, then steal it from them and build it myself. there's no law against it, it's just is kind of a dick move and seems kind of wrong.
as noted above, ceo is acting like a dick as well. but i think the way this dude is trying to play the victim through clickbaiting HN is a bit much. just my opinion.