It's interesting that the article doesn't have a comment to make about other familial arrangements brought about by shorter marriages and relationships.

For example, I'm older than my brothers, yet the oldest of them is not a middle child, because he's not a brother of my other brothers.

When families start spreading like this, a kid can be both a middle-child and a firstborn simultaneously - it becomes relative. And the attention that their position supposedly gives them becomes harder to measure.