Block design involves timed handling of components. Wouldn't SPD inherently impact the physical aspect of that?

Just a thought as one possible lens... given brain plasticity, if there is a deficit in one area of the brain, therapy or training can lead to an alternate area being involved in managing some sub-system. So, if his brain is being trained through OT to use his vision system to help manage his physical coordination, then the sort of impacts you are describing would make sense to me.