Dawkins' and Singer's arguments require one to believe that there is, ontologically and morally, no difference between taking the life of a fellow sentient creature and taking the life of another human being.

Depends what you mean by sentient. A human is more sentient than a turkey, which is more sentient than a carrot.

Therefore canabalism, and killing a human being to satisfy whatever physical, territorial, surval or supremacy need, is actually no more abhorrent than sitting down to a turkey dinner.

No, because of the difference in sentience.

At the most, it is wrong because protectiveness towards and empathy with members of our own species is somehow wired into us, and therefore it seems unfitting to fly in the face of that.

That is far too simple. We are more protective towards close relatives than others because of kin selection. But there tend to be limits to what we allow ourselves to do to others, but those change due to other influences - we can interact with others and change our minds. For example, in the past few centuries most of us have realised that all races are equally human.

It is a combination of wiring and culture; nature and nurture. We see changes in ethical standards in all cultures, no matter which God-given book they believe in, assuming they believe in any.