Except Ryokugyu and Kizaru have the same bounty and bounty scalers have always assured me that this is a valid way to powerscale. Kizaru was never said to be any stronger than Ryokugyu, all current Admirals are relative.
Once again Admiral fans are proven correct, Luffy’s amused reaction was only shown after Ryokugyu left and after he felt Celestial Dragon Shanks’ presence. The fact that Luffy stopped partying and brought Zoro, Jinbe, and Sanji to jump Ryokugyu shows that he was probably just as terrified of Ryokugyu as he is of Kizaru.