You're right, of course. If a hetero married couple discovered that the man was infertile, and they resorted to a sperm donor to have a child, the the husband would not have his parental rights revoked.
The rub in this instance is that the couple is going through divorce proceedings, so now the birth mother wanted the ex to be removed from the birth certificate. This is, basically, the most vicious custody dispute, ever.
Oh, and plot twist--the biological mom is now living with the biological dad! Gee, wonder what's going on here!
I do wonder if this child was conceived through legitimate sperm donation, or if it was more of an....au natural process. Because that kind of thing has popped up in the courts before. Generally speaking, a "sperm donor" is not recognized as a sperm donor by the courts if the "donation" was by actually having sex with the woman to be impregnated, even if all parties agreed at the time that the only purpose for the sex was to "donate". It has to all happen through legitimate medical procedure.