It’s only one biological mother at the maximum, out of that couple. That is what he is referring to. Even if there was a donated fertilized egg, that is still only one biological mother.
Easier said than demonstrated. Just look at all the legal boilerplate that has grown up around surrogate mothering just to make her think she hasn't got a chance if she decides to assert parental rights and I think your tune will be playing much more softly.
I adopted from the other side of the planet, because in America there is no such thing as a binding contract with a woman if a kid is involved.