If you'll look carefully in the video link that DML has provided, at time 3:52, you can see what appears to be Stratiolaelaps scimitus mites crawling all over the open cells. That could explain why this feral hive has managed to keep alive for the several years mentioned, without being destroyed by Varroa.
There is no doubt that the chemical treatments we have been using in our hives has changed the micro populations of the hives. After Varroa came and treatments began, "bee-lice" Braula ceaca, disappeared and so did the tracheal mites. I'm sure that other species also disappeared.
Having done our damage to the beneficial species in the hives, it is now our duty to try and re-introduce them so as to reap the benefits of biological control. The very least we could do is to try Integrated Pest Management (IPM) to get the best of both worlds and minimize the damages we cause with chemicals.