Actually, I doubt that he ever knew that.
obviously, he knows nothing about anti-ship missiles.
Beyond the obvious that he missed, like terminal homing maneuvers, dog-legs, etc, he also obviously doesn't know that AS missiles are designed to NOT detonate at impact, but several seconds later, after they've penetrated the hull, and explode INSIDE the ship.
I remember one harpoon vs target shoot onboard the USS Harold E. Holt, we weren't the firing ship, just the observers, but the target ship had been gutted, and was basically just a steel shell. The harpoon went through the ship, about 10 ft above the waterline, and exploded about 100yds, on the OTHER SIDE.
All that precious decoy would have done, is ensure that the warhead would have exploded closer to the real target.