Depends which Buddleia you mean - if its Buddleia davidii, you can take the tops off now to prevent windrock, and then cut right back to within 2 inches of the previous year's growth, so quite a hard prune. If its not davidii, those pruning instructions aren't right.

Practically all other Buddlejas are pruned hard right after they flower in spring, since they bloom on year-old wood.