The impact range of the 28cm rockets, like most rockets and HE ammunition, means nothing with regards to the amount of damage it is going to produce against a target.
400 yards is actually a bit close, I bet you SdKfz 251 crew felt the explosion blast being that close. They probably had to go wipe after that volley.
Also, remember that the 28cm rockets were used as a barrage weapon hence when fired they were fired at a general area, not an exact point. Be thankful if you got any splash damage on the tank to begin with. In AH, the accuracy variable is minimal with those rockets, so who knows.