Beauty Under the Moon
well first of all, i don't think the author understands the concept of surrogacy. with modern technology, the dad doesn't necessarily have to conceive the child through the sexual act, he just donate his sperm while his partner donate her egg. Then the zygote is implanted into the womb of the surrogate and is nourished by her body until birth. people who wants to have children through this method are usually the ones who can't conceive the children themselves, and they are not expected to cheat on their spouse for the sake of having kids.
The author and you are both right. Before the advent of IVF and artificial insemination, traditional surrogacy via nonartificial means existed although it generally was considered taboo then because it meant having actual intercourse with someone who isn't your husband. In that context then the author's use of the term surrogate can be justified. The fault here is that this story is written in our current time when artificial modern practices for insemination & gestation are already being used. Because times have changed then the meaning of the term surrogate mother has already changed. The generally accepted definition now of surrogate mother is exactly as you have said.