In the nebula, there would be no shields. But there would be no way to target anything much, either - Khan even missed a starship-sized target at twenty paces! The best defense against incoming torps in that soup would be to stay vewy vewy quiet and hope the enemy finds nothing to aim at.
I was actually referring to when they were chasing each other but before any ship enters the nebula. It's when Khan says, "Explain it to them."
The torpedo passed within a few feet of the warp nacelle pylon and under the saucer hull before detonating.