Ive made some comments about this already but I don't want to (personally) reduce it to "OMFG TRAITOR SHANKS!" because I feel like it might be going against what Oda is trying to do with Shanks overall. Like even if he didnt tell them about the fruit (im less confident in this atm, but we'll see), i do believe he talked to them about Luffy for some unknown reason, tied to some hidden motivation he has that relates to the endgame of the story. The story began with Shanks and entirely believe it ends with him too.
Yes, whatever it is, it wil probably some level of portrayal to Luffy. But I don't think he's some WG operative snitch. Something else very fucky is going on with the character when you put together what we know so far.
I'll also just leave this observation....
Vol 70:
Vol 76:
Vol 95:
Can you spot the similarity between each? Could be coincidence, could be intentional, who knows really...