Huh? Oda made it seem like the Gorosei didn't actually know about that until after Shanks presumably told them. That's why they were so flustered both immediately afterwards then tried to suppress all info about it by trying to get the form off the poster and downplay Luffy by spreading the Bounty increase an even 3 ways with Law and Kidd.
At least in MF the world and the marines at large saw that Luffy was made of rubber, but I don't know how many marines or Cp have seen him even in EL.
The fact that they didn't know would be even dumber and farfetched than their inactivity, especially given the fact that they were searching for that fruit.
Also, the theory that Shanks told them about Luffy has no basis, as of now; there isn't a single reason for Shanks to betray Luffy. Shanks betrayed him the second before and than stopped Ryokugyu who was going after Luffy? Why? No reason, as of now.