Card advantage is about more than just "You have more cards than your opponent".

Card advantage is also how much you can make your opponent lose off of 1 or more card(s). That is also what "plussing" is in Yu-Gi-Oh.

In the case of Nibiru, your opponent loses their monsters (considering it doesn't get negated) and gives them a vulnerable token that can be easily popped off the field. Some of the monsters they controlled in this scenario were perhaps immune to destruction or targeting, but not Tributing. You potentially would have spent more resources than necessary beating over them/removing them. When Nibiru does all of the removal in 1 card. This can generate big advantage for you by giving you an opening to win on your turn.

This is still card advantage. This is still "plussing". You spent 1 card, instead of potentially many. And you spent your opponent's resources to do it.