Not really such a thing as "equal" in a relationship. Relationships are never truly equal, one person will be putting in more than the other and it flips back and forth throughout the tenure of the relationship. Communication is key in those situations.

One person will be more Dominant than the other, same with submission, one will be more submissive than the other. Unfortunately these terms have become heavily conflated with a negative bias. You're conflating Dominance with Domineering - these are two different things.

In the case of this article it is potentially referring to Masculinity and feminity (Masculine being more Dominant and feminity being more submissive)

If you choose not to believe me, especially if you don't see it in straight relationships. Go look at lesbian or gay couples. There's always a "butch" aka masculine (dominant) woman and a feminine (submissive) women. Same with gay men one is more masculine the other feminine.

Dominance and submission as with Masculinity and feminity are universeal laws in this world, you can try and get rid of them, they WILL be around in some shape. One person will become the Dominant and the other the submissive (which person will become which, who knows)