You have a good point. The larding step is done in regular prosciutto, but that has a curing time of at least 9 months and the lard is quickly absorbed by the meat. I don't think the curing time for duck prosciutto would be long enough for this to happen and it's likely that it would just prevent exposure to air. Also, unlike regular prosciutto, duck prosciutto isn't treated with sodium nitrite; preventing exposure to air on the surface of the meat could raise the risk of botulism.