how does that even make sense

It's just the usual uneducated racism..

Belgium does not have birthright citizenship, meaning that not everyone born in Belgium is a Belgian citizen

So the statement is correct

Nothing wrong with that, the world is not lala land with pink fart ponies, their country their rule. If a cat is born in a kennel that doesn't make it a dog or if a man is born in a garage that doesn't make him a car.