Sure Shanks displayed an impressive ability to paralyse Greenbull. Probably some AdvCoC of some kind. However GB retreated due to having to fight the entire crew....not Shanks alone.
The 2 pieces of Dialouge he mentions them.... he includes the entire group and not Shanks alone.
Not one statement or mention of Shanks himself. Just the RHPs as a crew. And yes the RHPs would stomp Greenbull. An Admiral cannot defeat a Yonko with their entire crew alone. Makes zero sense.
This chap was a great haki feat for Shanks. But does not imply Shanks 》GB or GB retreated due to Shanks alone.