Because he said "I am not saying that we’ll have science that can answer every conceivable moral question". His point is, in my understanding, don't exclude science when we talk about moral issues because some moral issues can be explained by science. If it is his point, I agree with him. But for a few fundamental, universal moral values, can science explain them? I doubt about it. science is about fact, a "is" issue, morality is about value, a "ought" issue. You can't derive "ought" from "is". 300 years' old Hume's law will still be valid.