The cattle were enclosed in the early 1300s as means of shooting/hunting/food source, but never tamed or handled by humans. That made them safe from the (Scottish) Border Reivers in what were fairly lawless times - they could not be herded and driven away. They are fenced in by stone walls and the flock has never been added to nor have any animals been taken away for breeding somewhere else. These are wild cattle!
That also means that the genetic base is developed from what it started out before any real improvements in animal breeding were made by man, and the fact that there was (and still is) no selection of breeding stock, the strongest bull is in charge for a period of about 3 years and mates all the females. Female calves do not breed until they are about 3 years of age, so father/daughter matings are very rare. Genetically all animals are identical, and bear no resemblance to any other catte breed in the world. They are not massive, have strong shoulders, smaller rear ends (so they can run fast!), calves are born very small (and no difficulties in calving, which happens all year round) and lead bull decides if the calf is accepted by the herd. If he thinks it's not, it is either abandoned by the mother or killed by the bull - weaklings do not survive.
Have a look at their website!
But with regard to other rare breeds - we often keep all animals that can survive with human help, breed from them etc etc. It is often said that prolonged in-breeding a group of animals wll produce a smaller, weaker animal that needs outside blood to re-invogorate it. However these cattle have never been added to, so what's all that about? Human intervention etc etc.
It is just interesting, but it's an experiment that cannot be carried out by anyone on any other rare breed.... after all who can plan nowadays for 800 years ahead...