I was just posting of what I felt and how it worked for me. It helps when I add new ducks to my flock. Weather it does or doesn't cover cents, its was meant to be "as if it does". In my perspective I just like to think it does. When I try to add new ducks without it, they pull on each others feathers and cause bleeding. But when I put VetRx on everyone and the new ones, they get along fine and no fighting. I was simply stating how well it works for me.
Short of respiratory and sinus issues, it won't help with that much. Birds don't sniff each other out, so it's not going to "cover up scents" for flock integration though rubbing menthol products all over everyone's faces might just give them something else to be thinking about. Put Icy Hot on your face and see if you still feel like fighting. Their beaks are very sensitive, so it might even burn.
Birds have very good senses of sight and hearing but can't smell or taste well. Menthol doesn't strictly act on taste and smell though, which us why it can still affect birds by clearing sinuses just like it does with us. With mammals (and presumably birds as menthol products appear to have the same effects) menthol actually triggers cold receptors, just like capsaicin trips heat sensors (and both trip pain sensors). Thus, the bird doesn't need to smell or taste anything with menthol because it's whole body (just like ours) is sensitive to cold. Given Muscovy are particularly sensitive to cold (being tropical non-migratory birds) they might even be extra sensitive to method compared to mallard derived breeds which are far more cold tolerant