Chewy meat on poultry (duck) bones
In terms of food safety, temperature is the best and most reliable rule. 165F is good enough for duck, so yours is definitely all safe to eat. No need to pitch it, unless you think the texture is so bad you don't want to eat it.

Pulling away from the bone definitely isn't a perfect food safety test. You'll definitely be able to get poultry to a safe temperature without the meat pulling easily and cleanly off the bones. So yes, if by overcooked you mean cooked beyond the minimum safe temperature, or held at that temperature for a long period of time, that test will cause you to overcook it. (I don't think it goes the other way - it's not going to pull away from the bone if it's under the safe temperature.) But at the same time, you might prefer the texture when it's pulling away from the bone, so it doesn't matter if it's beyond the minimum safe temperature. If you love the results, it's not really overcooking, is it?

But it sounds like yours was perhaps all overcooked by any definition - both chewy and dry are generally signs of overcooking, and you got 10-15F above the minimum safe temperature too.