I see the point that there is no single way to separate the unborn from its mother, while in all other cases there are ways that it can survive when separated (even though the separation might not be allowed).
I would say for full consistency with this it would imply abortion providers ought to take steps to not directly inflict harm, but practically speaking death is assured under this framework and so perhaps that falls onto questions of suffering vs painless death.
Edit: To be more direct, the killing is considered acceptable since the alternative is less acceptable: namely, that a person who is providing biologically, metabolically bound life support to another via an umbilical cord is not permitted to cut that cord of their own agency, under penalty of law.