While I put Yonko > Admiral myself, I do think there's more to Shanks than just being strong to be able to scare away Greenbull.
Sengoku also made a comment that he only stops the war because it was Shanks asking, and the Gorosei also allow him to enter their chambers.
I think Shanks being a celestial dragon and maybe even directly related to the Gorosei could clear this up. Admirals are directly under command of the Celestial dragons, so it would make sense for Greenbull to leave Wano when Shanks orders him to.