I agree with the wildlife expert. From everything I've read, foxes will not tackle a full-grown cat. If they see feline road-kill, or a cat who has died from other causes, they will take the body. This is probably what the owner who saw his cat dangling from the fox's jaws was actually seeing: his already dead cat being scavenged by the fox.
A fox will attack small kittens or cats who are significantly weak; but a healthy adult cat is too much of a challenge: too large and armed with claws. Only two dogs (canine) working as a pack can corner a cat and kill it. Foxes hunt alone. Cats are efficient climbers; foxes aren't.
Personally, I think the "a fox killed my cat" idea is an urban myth, or just a mistake based on half the story (cat in fox's jaws). Regarding Mr Fox, our cats are relatively safe imho, however, cars are another matter altogether.