Do you mean this?
An act of love is sufficient for the adult to obtain sanctifying grace and to supply the lack of baptism; to the still unborn or newly born this way is not open.
Pius XII doesn't define act of love there so I don't think you can accuse him of error.
Does he go beyond the Roman Catechism for example? This says:
On adults, however, the Church has not been accustomed to confer the Sacrament of Baptism at once, but has ordained that it be deferred for a certain time. The delay is not attended with the same danger as in the case of infants, which we have already mentioned; should any unforeseen accident make it impossible for adults to be washed in the salutary waters, their intention and determination to receive Baptism and their repentance for past sins, will avail them to grace and righteousness.
I would not be comfortable on what he said to claim he does.