I don't mean to be rude at all and you are allowed to do anything with your mallards. You probably took care of them for months or years and know what you are doing so it can't really go wrong. I know the law doesn't charge you if your mallards willingly leave your farm or backyard as you are not mandated to cage them. Yes, they might be tasty, but you can get a wild mallard (farm raised) at a supermarket for $11.25 per duck. We probably waste $11.25 on cable every week for 50+ weeks every year. Just think about letting them fly away as a donation to your local area the wild. I mean, do you really want to butcher your own ducks? Its possible, but doesn't really settle the cost benefit equation. If they are mallards, there is no possible way that you will be held responsible if they choose to fly away on their own free will. They are voting with their feet and we cherish our right to move if the circumstances change.
I found the website advice are questionable as best. I know its attractive to throw the rescue label and every word turns true, but the website's underling logic is confusing at best. They prefer 100% fatality by steel instead of possible survival in the wild. If they are domestic ducks, this might be true and an attractive idea possibly, but they are wild mallards. They have learned to trust you and do not see you as a predator or themselves as food. If they didn't trust you, they would have flew away. They might have even imprinted on you. My ducks can fly (learned today) and they allowed me to pet them (they usually don't allow it and they run away from strangers). I think it would be the cruelest of ironies to be slaughter and butchered by someone that the mallards knew for all of their lives. I won't make your decisions, but I hope I was able to stimulate the topic a bit for you. There is a chance that a wild animal can get them, but it is vastly overstated by propaganda groups. The resident population of any Mallards, Canadian Geese, etc are the testament to their ability to survive in the wild if they are genetically a wild species. In some warm places in the South, even Pekins can surive in the wild! There is a Youtube video of it.
Ultimately, it is your choice. I wouldn't do it, but everyone lives under different circumstances.