I'm afraid I've never heard someone try to argue about the difference between calling it the zygote stage of human life and a human zygote. It's just two ways of labeling the same thing. I don't know that I've ever seen or heard someone deny that the zygote is a stage of human development. If I'm reading this right, you are trying to compare a zygote to a newborn as though they are the same. Certainly they are the same organism at different stages of growth, but there are massive anatomical/physiological differences in the two stages.