Can any creature be knocked out?

Reading the question Can you knock a Banshee unconscious?, I see that mu is too short’s answer quotes the rules from the Player’s Handbook on knocking a creature out, saying that:

When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out.

Furthermore, it claims that since the creature asked about isn’t immune to unconsciousness, it can be knocked out.

Is this really true? It seems quite counter-intuitive, especially for incorporeal creatures (e.g. banshees or ghosts, neither of which have a listed immunity to unconsciousness).