My interpretation of Greenbull introduction is that Sanji and Zoro will fight Admirals.
Basically it says "this guys are way stronger than commanders, so when our protagonists clash with then you need to feel the tension"
I mean, everyone knows that Luffy-Law-Kid can at least beat Greenbull on a hard fight. His intro with commanders, to me, meant that Admirals are something that Luffy top guys need to surpass in the future for their captain