This is all about composition over inheritance. What if I want a sword that has a flask built in? How do I reconcile the fact that Potion and Weapon are two different branches of the inheritance tree? His points about pushing things out to a data file are also very valid.
Granted, the author may have taken it a little far, but the sentiment is right.
What you describe is a very special and exceptional case to have it affect everything else. Personally what i would do in such a case is to have the Weapon object own a WeaponFlask object and when the weapon is added/removed to the player's inventory it automatically adds/removes the WeaponFlask object too and the WeaponFlask itself can veto being removed from the inventory. You'd need to add extra events for that, but assuming we're talking about an RPG style game, you most likely will have functionality for stuff like that anyway for other items (e.g. cursed items that cannot be removed, enchanted items that apply/remove an Effect when they are added/removed from the inventory or the player wears them, etc).
Granted, the author may have taken it a little far, but the sentiment is right.