The stories about roosters fighting off Grizzly Bears, Killer Whales or other predators are similar to the fairy tales about DE fighting mites. They are just old wives and husbands tails. That is why the LOVELY, AMAZING ,or AWESOME, (I chose to use a different adjective) Great Horned Owl is such a fearsome predator of tree roosting free range chickens because after the Sun goes down chickens are totally at the mercy of every chicken killing varmint known to man.
I agree with you about the roosters, most roosters won't fight big predators, but the DE fighting mites? That is true, as far as I know. No old wives tale there. I had an outbreak of mites not too long ago and DE shut it down real quick. I covered my birds and covered my coop area. I haven't seen a since mite since. I've also taken in a sickly silkie from a run down feedstore in my area and she was covered in red mites and she was very pale. After a few days of putting her on vitamins and bathing her and coating her in DE, she continued to live just fine and haven't seen a mite on her since.