"Overall, lamb losses to foxes are very low, and fox predation is much less important than other causes of lamb mortality. Higher than average levels of lamb mortality are associated with poor standards of husbandry, and so the most cost effective way to reduce lamb losses is better husbandry."

A very sweeping assertion, no evidence provided to back it up.
Lamb mortality can be from many varied causes. The cause under discussion is foxes and as you state they kill surplus prey. The other causes are hardly relevant to this particular discussion.