I wonder if it's not more the action of the dog. Again, from my experience, coyotes in a pack will attack an animal they consider to be either a minimal or no threat (weaker dog on a chain for example) or an animal that is an immanent threat (charging dog, for example). I have never personally heard of, or experienced, a pack actively hunting a strong, free running dog. A single coyote is a coward, and would never consider going after an animal that may harm it. A pack is driven by need, and has more courage...the strength in numbers thing, but is still unlikely to actively hunt something that can fight back and cause damage. However, if the pack finds an advantage...all bets are off.
They are very intelligent animals though. Trap just one of a pack and that trap system is useless on the rest of the pack. They learn fast. I used to get $10 a hide from local ranchers and baited coyotes sometimes to draw them in. They even started avoiding the bait (chicken and fish).
I don't doubt that they have taken dogs (cats are easy for them), but actively hunting a free roaming dog just doesn't seem to be in their nature.