It's simply a case of appropriate measures. If someone really wants to steal your bike, there's little you can do to stop them, but you take appropriate measure to make sure it's difficult enough, so they'll move onto to something easier.
In a low risk environment, the middle of off-road cycling across Dartmoor for example, you probably wouldn't need to use a lock at all, since in the middle of the night it would be sheer luck to find you at all, let alone target you for a theft.
In most countryside campsites, probably just locking the bike up, or preferably to something relatively immobile ie a fence or picnic table should be more than adequate. If you're using a campsite which is a bit more urban, than you need to find a less attackable object, such as a real Sheffield stand, or something else solid and metal.
Not using your best carbon fibre racing bike, or desirable expensive full suspension mountain bike is also advantageous. Nothing supposedly turns off your modern bike thief like drop handlebars, although a poorly locked expensive Ti bike could still be swiped by a knowledgeable opportunistic thief.
I suspect ground anchors although they sound like a good idea, would practically be far to much of a faff to use, provide little additional security, and quite likely be far too heavy to carry, or if reasonably light be made of the finest cheese.
