I believe the First liked to give the impression that it was Omniscient, but was far from. It will have known what Buffy had said because it eaves dropped. Maybe it even knew about the Slayers, but lets face it, why should that bother it...an Army(Ok send 25 troops too Baghdad and see if that equates an ARMY) of slayers, around 25 plus the others, your looking 35 personnel in total, against an Army of thousands of not Vampires BUT Turok-Han....i see no reason to be worried. The Downfall to the Firsts Plan was that instead of Following Buffy it Followed Spike, so it didnt get the full dealy on what the Amulet could do.....Plus The First interuppted with the plans of The SP's didnt it......so they may have masked knowledge of what the amulet can do.....obviously this is just supposition, but i cant believe(my world will literally fall apart if i do) that Joss would allow such a gaping hole in something he missed but FANS picked up......i mean this isnt Xander holding an egg-cup in one hand then cut back and its a razor blade..........