This is tied to YAGNI. Do you really need to impose this restriction on this database? Don’t add restrictions “just in case it might be useful”. If you don’t have a good reason, you ain’t gonna need it.
I think they're saying that e.g. you shouldn't index a natural key unless you know that you're going to search/collate by that key as a column. Telling the database that a certain column contains (a component of) the primary key is adding a restriction to that column.