Maybe for the DB domain author is talking about but the nice thing about a bool is that it's true or false. I don't have to dig around documentation or look through the code what the convention of converting enum, datetime, etc. to true/false is. 1970/1/1 (I was four years old then, just sayin), -6000 or something else?

Nullable helps a lot here but not all languages support that the same way.