Seriously ask yourself this. 1000 chapters have gone by and the Gorosei never utter concern about actually eliminating Luffy or his powers. During CP-9, it was always about Robin. They even compromised with her so that they could live and she would die. Even here, it was about Robin entirely. Any other time, it was in the context of the Marines trying to kill him due to his dangerous lineage
Now, very clearly because they figured something out about his powers, they entirely shifted the CP-0 goal to killing Luffy
I'm telling you, Shanks told them something about Luffy they didn't know. They rolled with it until something changed at Wano (i.e. maybe Zunisha) or some offscreened conversation.
Again, I don't presume to know WHY he did this (only guesses), but i think this definitely supports Shanks talking about Luffy to them, especially since he only went to them after his new bounty.
It makes you wonder why the whole Gomu/Shanks/Nika thing even happened with WsW and Jinbe. Seemingly throwaway plot sprinkled into the narration that in hindsight might have been huge foreshadowing