I think the point is that they consider even cheeses like mozzarella, which are harder than cottage cheese but not normally considered a hard cheese for the purpose of waiting 6 hours, as fully hard cheeses
If I may chime in on this topic, while it does seem that Kedassia requires 6 hours after pizza or mac and cheese, what is less commonly known if that pizza and mac and cheese in England is usually made with cheddar cheese rather than mozzorela, I think most Rabbis around the globe agree on 6 hours for cheddar cheese.