In the end, they fought YCs. We all know Zoro plays the middle ground with Luffy and Sanji. Zoro is the type to occasionally fight Luffy's opponents (see Arlong, Enel, Lucci, Hody, and Kaido), and even get unique moments against secondary antagonists (Mihawk and Kuma).
But at the end of the day, he's aways paired off with Sanji to fight the #2 and 3 of an arc.
If Luffy fights the Fleet Admiral, they will likely fight the Admirals. It's just how it works.