2.5 years sounds pretty damn short for a clean-sheet (no existing assets), full-price, multi-platform game with story and multiplayer on a latest-generation FPS engine. Presumably they already had most or all of the engine developed at that point, otherwise that would be a ridiculous time-frame.
Most clean-sheet AAA titles I'm familiar with (HL2, Doom 3, Rage, UT3, Max Payne 3, Bioshock Infinite) took at least four years.