Thanks for the reference.  It seems like the distinction banished was between the protection afforded the quickened, vs un-quickened, fetus's. A moral distinction. One may find support in that, for the view that the soul comes at conception, but it falls well short of making that assertion. It's an argument from moral caution, rather than certainty.

From the EPE:
"Pope Pius IX challenged the canonical tradition about the beginning of ensouled life set by Pope Gregory XIV in 1591. He believed that while it may not be known when ensoulment occurs, there was the possibility that it happens at conception. Believing it was morally safer to follow this conclusion, he thought all life should be protected from the start of conception. " 

I know it's hard to believe, but one line of the Church's argument rests, in significant part, on moral caution, rather than material certainty. Another rests on equivalence -- that to kill the thing at point A is morally the same as doing so at point C or Z