Framework - Plan Modifier Documentation Example

I am looking at the example plan modifier shown in the documentation which should apply a default value to an attribute when it is not configured. The comment mentions that the attribute must be marked as Optional and Computed. Though from what I have observed the planned value of a computed attribute is never null and therefore the default value never applied. This is somehow confirmed by the following statement in the documentation here:

Computed attributes, whether optional or not, will never be null in the plan for Create, Read, Update, or Delete methods.

Am I misunderstanding something here or is the example just not working as intended?