If it's Imu you have to ask why didn't he alert the gorosei earlier? They carry out his will. Also considering they say they try to capture it in every era i don't think they need shanks to inform them of that or Imu
It's more the question of why they were concerned until the moment zunisha arrived.
Like look at the 1037 dialogue on the last page It starts with the dreadlocks Gorosei saying "No...that's impossible!" As if they don't believe the rumor about the devil fruit.
Something is definitely off here, which is why I think we lack context as to how or why they are concerned about it right now.
I still think Shanks told them something, but only at 1041 did I think it could be about this. For example, maybe he told them that Luffy has it and believes he is getting close to awakening it, something they didnt believe, until they saw Zunisha, Joyboy's companion, show up.
But even that is weird, because it doesn't seem like Zunisha is really public knowledge (or a threat) to anyone. Like, this whole time, they've never talked about the elephant, only Luffy
Idk, I'm more confused than anything at this rate lol. Oda needs to explain why they became concerned only after 1037. The page before on the same chapter, they were still talking about capturing Robin