Hello Dennis

Sorry for the delay in answering your question.

Insecticidal soap is a contact insecticide, meaning it must contact the target insect- there is no residual effect on insects that hatch later.

Also, insecticidal soaps are not very effective on caterpillars which is the box tree moth life stage that damages boxwoods.

“Most research with insecticidal soaps and detergents has involved control of plant pests. In general, these sprays are effective against most small, soft-bodied arthropods, such as aphids, young scales, whiteflies, psyllids, mealybugs, and spider mites. Larger insects, such as caterpillars, sawflies and beetle larvae, generally are immune to soap sprays. However, a few large insects, including boxelder bugs and Japanese beetles, are susceptible.” — from https://extension.colostate.edu/topic-areas/insects/insect-control-soaps-and-detergents-5-547/

Effective treatments are the ones recommended in the article you read specific to box tree moth. Use a product labeled for control of caterpillars, and please use exactly as the label directs, following all precautions. 
https://www.canr.msu.edu/resources/insecticide-options-for-box-tree-moth-management-ir4-project