Letting the app code enforce these constraints isn't ignoring the problem, it's how you solve this. Your DB is never going to represent all the business logic by itself. You can also add the slightly clunky constraint it mentions if you really want.

I wouldn't do this with separate tables. I also wouldn't do this with polymorphism, or OOP in general, even if the DBMS properly supported OOP. Trying to represent these constraints by classifying things will get confusing fast.