Milah/Geirus of Eved K'nani 2

In the previous post we brought the Rambam that says that if a person buys a shifcha k'nanis who is pregnant, the baby has a din of a yelid bayis and he is nimol l'shmoneh. However, the Rambam says that despite this fact, if the shifcha is toveles after the baby is born, even though the baby is nimol l'shmoneh, still the milah is not docheh shabbos. The question is why.
The Grach (Milah 1:10) in one attempted answer suggests that in this case, even though the milah is done on day 8, still the baby is currently a non-Jew (because his mother wasn't toveles before his birth), therefore a non-Jew's milah can never be docheh shabbos.