You gotta remember that the Emperors have their crews. While each Admiral can match an Emperor, it's going extreme diff at the very least regardless of who wins. You still gotta deal with the commanders, the veteran level fighters, and their allies. The Warlords would match one Emperor and their crew, the 3 Admirals would deal with the other 3 Emperors, and the Fleet Admiral and the rest of Marines deal with the remaining 3 Yonko crews. Just my interpretation of it if that helps you out. No implication of Yonko > Admirals in the statement of Navy + Warlords = Yonko statement.
I actually meant 4 emperors crews.
Will edit my post.