I dont think this is a gag!!
All people around Zoro though his fury cause the earthquake but we know that Kaido cause it. Still the comment about Supreme King hacky and Brook affirming that Zoro is a beastly man isnt something new!! Many people around Zoro do feel this beastly aura around him. First Apoo, then in Punk Hazzard vs the dragon, Monet, the sumo guy and now all people around Zoro in onigashima,
Zoro said clearly that he didnt cause the earthquake but he didnt said I dont have CoC which make sense!! How many CoC users cause earthquake so far? None!! The fact that even Brook was confused and though Zoro used CoC does show that Zoro could have already use it in the past!!
I really like it that Robin affirmed that she know how Zoro feel, which implied a deep connection between the 2!!!