...which is not the same thing as denying God. An agnostic is open to the possibility of God.
However, there is a sense in which one must make a decision about God one way or the other. This is different from something like, say, being open to the possibility that there is life on Mars. For if one is agnostic about life on Mars, his mindset is such that he could accept there being life on Mars and he could accept there being no life on Mars. Either outcome is in some sense OK with him.
But with belief in God it is different. At least the belief in a Christian God is different. That is because if one does believe in such a God, he would be compelled to at least try to live his life in accord with that belief, by developing a relationship with God through prayer, etc. But if he lives his life as if there is no God, he has made the decision in his mind that such a God does not exist, for the existence of such a God would demand a response. This is different from the life on Mars question because belief in life or non-life on Mars does not demand any specific response. But belief in God does. Since a person can either have a response or not have a response, there is no middle ground in the same way there is as middle ground on so many other questions. So I doubt DeGrasse-Tyson's sincerity when he says he is merely an agnostic. By his choice not to respond to God, I think that in his heart he is an atheist.