In fact he answers his own question a few lines later:
<<Tied to the notion of upsherin (from the same root as the English "to
shear") is the idea that this is the beginning of the child's education. He
is taken to learn aleph-beis on that day. This again implies a
boys-learn-Torah association, suggesting a reason why girls don't have an
upsherin, but the verses don't imply anything about this second
custom.Also, with the first haircut comes the first opportunity to keep the
mitzvah of not cutting peiyos(side-burns).>>
Since the mitzva of preserving the peiyos applies to males only, it follows
that the custom of removing the rest of the hair is observed for boys only.
In fact this mitzva is given in the same parsha as the Orla (fruit of the
first three years) on which the quoted Zohar is based.
By the way, upsherin (opsherin if you are a litvak) is just the Yiddish for
having a haircut. In some circles the custom is not called upsherin but
"makhn peiyos" - 'creating sidecurls.'