I 've never heard the interpretation you say the Catholic apologist gave. It sounds like pure speculation to me.
Here’s what a This Rock article has to say about it:
In Paul’s first epistle to the church in Corinth, he treats a number of subjects. This letter was written to counteract problems he saw developing in Corinth after he had established the church there. Corinth had its share of pagan religions, but there were also quasi-Christian groups that practiced variations of orthodox Christian doctrines. Enter baptism for the dead.
Mormons cite a single biblical passage to support baptizing members on behalf of dead persons, “Else what shall they do which are baptized for the dead, if the dead rise not at all? Why are they then baptized for the dead?” (1 Cor. 15:29).
Mormons infer that in 1 Corinthians, Paul speaks approvingly of living Christians receiving baptism on behalf of dead non-Christians; however, the context and construction of the verse indicate otherwise. The Greek phrase rendered by the King James Version as “for the dead” is huper ton nekron. This phrase is as ambiguous in Greek as it is in English. The preposition huper has a wide semantic range and can indicate “for the sake of,” “on behalf of,” “over,” “beyond,” or “more than.” Like the English preposition “for,” it does not have a single meaning and does not require the Mormon idea of being baptized in place of the dead. Such a reading would be unlikely given the more plausible interpretations available, and even if huper were taken to mean “in the place of,” it doesn’t mean Paul endorses the practice.
First Corinthians 15 is a key chapter for Paul’s teaching on the resurrection of the body. He makes no statement on baptism for dead persons except to note that some unnamed “they” practice it. While the rest of his teaching in chapter fifteen refers to “we,” his Christian followers, “they” are not further identified. Who this group was may not be known with certitude today, but there are some reasonable interpretations:
(the article then gives four alternative interpretations, none of which refer to the interpretation at issue)
There is no other evidence in the Bible or in the early Church Fathers’ writings of baptism being practiced on the living in place of the dead. Some Mormon writers assert that some Christian commentators have discussed the possibility of a kind of “baptism for the dead” among some in the Corinthian community in Paul’s time. But these commentators do not suggest that the practice was accepted or mainstream. Given the silence of Scripture and tradition, we conclude rightly when we see this behavior as another aberration within a community of believers already soundly scolded by Paul for its lack of charity, its factionalism, its immorality, its abuse of the Eucharist, and other matters.
Although we have no way of knowing for sure who was engaging in this practice, it is certain that Paul was not referring to orthodox Christians baptizing the dead. Catholic and Protestant scholars agree on that.
To see the entire article, go to the Catholic Answers home page and type in “Mormonism’s Baptism for the Dead” into the search bar.
