Given your appropriate efficient -market hypothesis quote, it is not necessarily the case that the divergence in ABBV and ENTA price means the market thinks the overall size of the HCV market has decreased.
More to the point, I think the market today assumed ABBV gave away the store. We have no evidence of that though. Just as the market is not always efficient, neither are CEOs. I saw Miller at an HCV task force, where Gilead was also present. He had some real venom for them.
Another scenario is Miller did a deal to try and screw Gilead in any way he could, but ABBV did not give away the store.
Again, we do not know the size of the discount ABBV gave. Are they stupid enough to capitulate this early in the game?
