The article probably means: Define anything as non-nullable which can be non-nullable. Unfortunately SQL defaults to nullable, so there is a tendency to define too many columns as nullable. Normalization can also reduce the need for nullable columns in base tables (but you will get them back if you perform an outer join, so it is not a panacea).

But if a columns truly has unknown values, NULL's are the best ways to represent it. It is sometimes suggested to use "sentinel values" like empty string or -1 to represent missing values, but IMHO this is much worse than NULL's, since these will be treated as regular values by operators. When you have missing values, you want three-valued logic.