Why would he do that when science and embryology says otherwise and he is certainly more qualified than you on the matter says.
"The development of a human being begins with fertilization, a process by which two highly specialized cells, the spermatozoon from the male and the oocyte from the female, unite to give rise to a new organism, the zygote."
[Langman, Jan. Medical Embryology. 3rd edition. Baltimore: Williams and Wilkins, 1975, p. 3]
Embryology is not the "Domain Science". Regardless if "ANY" scientist or textbook wants to claim that the zygote is a living human then they need to give an explanation of why this is so.
The above quote does no such thing.
Further .. saying "the development of a human being begins here" does not mean a living human exists at that point.
Being in a phase of product development does not mean that the product exists.
Regardless. There is no "WHY" this claim is true given. No reasons. It is a naked claim.