there are certain halachas regarding who is considered a virgin.
a girl who had sex after 3 is no longer a virgin. but if she had sex before then (by rape or whatever), the virginity grows back and she retains her status as a virgin and can later on marry a high priest who is required to marry a virgin.
as for that quote, in those days, there was great promiscuity among the gentiles and so any woman who comes to convert was assumed to have committed a forbidden sexual relation regardless of her age.
but sex with a girl under 3 is not considered sex since the virginity grows back. so even if she was raped by a nutcase such as the one who wrote that article, then she is permitted to marry a priest, who is required to marry a woman who never had forbidden sexual relations.
the verse of midian refers to keeping alive the girls who never reached this status.